diff --git a/src/Tokenizers/PHP.php b/src/Tokenizers/PHP.php
index 2e8c87bed3..03096887bb 100644
--- a/src/Tokenizers/PHP.php
+++ b/src/Tokenizers/PHP.php
@@ -992,20 +992,24 @@ protected function tokenize($string)
 
                     if ($tokens[$i][0] === T_LNUMBER
                         || $tokens[$i][0] === T_DNUMBER
-                        || ($tokens[$i][0] === T_STRING
-                        && $tokens[$i][1][0] === '_')
                     ) {
                         $newContent .= $tokens[$i][1];
+                        continue;
+                    }
 
-                        // Any T_DNUMBER token needs to make the
-                        // new number a T_DNUMBER as well.
-                        if ($tokens[$i][0] === T_DNUMBER) {
-                            $newType = T_DNUMBER;
-                        }
+                    if ($tokens[$i][0] === T_STRING
+                        && $tokens[$i][1][0] === '_'
+                        && ((strpos($newContent, '0x') === 0
+                        && preg_match('`^((?<!\.)_[0-9A-F][0-9A-F\.]*)+$`iD', $tokens[$i][1]) === 1)
+                        || (strpos($newContent, '0x') !== 0
+                        && substr($newContent, -1) !== '.'
+                        && substr(strtolower($newContent), -1) !== 'e'
+                        && preg_match('`^(?:(?<![\.e])_[0-9][0-9e\.]*)+$`iD', $tokens[$i][1]) === 1))
+                    ) {
+                        $newContent .= $tokens[$i][1];
 
                         // Support floats.
-                        if ($tokens[$i][0] === T_STRING
-                            && substr(strtolower($tokens[$i][1]), -1) === 'e'
+                        if (substr(strtolower($tokens[$i][1]), -1) === 'e'
                             && ($tokens[($i + 1)] === '-'
                             || $tokens[($i + 1)] === '+')
                         ) {
@@ -1019,9 +1023,21 @@ protected function tokenize($string)
                     break;
                 }//end for
 
+                if ($newType === T_LNUMBER
+                    && ((stripos($newContent, '0x') === 0 && hexdec(str_replace('_', '', $newContent)) > PHP_INT_MAX)
+                    || (stripos($newContent, '0b') === 0 && bindec(str_replace('_', '', $newContent)) > PHP_INT_MAX)
+                    || (stripos($newContent, '0x') !== 0
+                    && stripos($newContent, 'e') !== false || strpos($newContent, '.') !== false)
+                    || (strpos($newContent, '0') === 0 && stripos($newContent, '0x') !== 0
+                    && stripos($newContent, '0b') !== 0 && octdec(str_replace('_', '', $newContent)) > PHP_INT_MAX)
+                    || (strpos($newContent, '0') !== 0 && str_replace('_', '', $newContent) > PHP_INT_MAX))
+                ) {
+                    $newType = T_DNUMBER;
+                }
+
                 $newToken            = [];
                 $newToken['code']    = $newType;
-                $newToken['type']    = Util\Tokens::tokenName($token[0]);
+                $newToken['type']    = Util\Tokens::tokenName($newType);
                 $newToken['content'] = $newContent;
                 $finalTokens[$newStackPtr] = $newToken;
 
diff --git a/tests/Core/Tokenizer/BackfillNumericSeparatorTest.inc b/tests/Core/Tokenizer/BackfillNumericSeparatorTest.inc
index bfdfcb9ac4..eef53f593b 100644
--- a/tests/Core/Tokenizer/BackfillNumericSeparatorTest.inc
+++ b/tests/Core/Tokenizer/BackfillNumericSeparatorTest.inc
@@ -1,10 +1,14 @@
 <?php
 
+/*
+ * Numbers with PHP 7.4 underscore separators.
+ */
+
 /* testSimpleLNumber */
 $foo = 1_000_000_000;
 
 /* testSimpleDNumber */
-‪$foo = 107_925_284.88;
+$foo = 107_925_284.88;
 
 /* testFloat */
 $foo = 6.674_083e-11;
@@ -12,14 +16,67 @@ $foo = 6.674_083e-11;
 /* testFloat2 */
 $foo = 6.674_083e+11;
 
+/* testFloat3 */
+$foo = 1_2.3_4e1_23;
+
 /* testHex */
 $foo = 0xCAFE_F00D;
 
 /* testHexMultiple */
 $foo = 0x42_72_6F_77_6E;
 
+/* testHexInt */
+$foo = 0x42_72_6F;
+
 /* testBinary */
 $foo = 0b0101_1111;
 
 /* testOctal */
 $foo = 0137_041;
+
+/* testIntMoreThanMax */
+$foo = 10_223_372_036_854_775_807;
+
+/*
+ * Invalid use of numeric separators. These should not be touched by the backfill.
+ */
+
+/* testInvalid1 */
+$a = 100_;   // trailing
+
+/* testInvalid2 */
+$a = 1__1;   // next to underscore
+
+/* testInvalid3 */
+$a = 1_.0;   // next to decimal point
+
+/* testInvalid4 */
+$a = 1._0;   // next to decimal point
+
+/* testInvalid5 */
+$a = 0x_123; // next to x
+
+/* testInvalid6 */
+$a = 0b_101; // next to b
+
+/* testInvalid7 */
+$a = 1_e2;   // next to e
+
+/* testInvalid8 */
+$a = 1e_2;   // next to e
+
+/* testInvalid9 */
+$testValue = 107_925_284 .88;
+
+/* testInvalid10 */
+$testValue = 107_925_284/*comment*/.88;
+
+/*
+ * Ensure that legitimate calculations are not touched by the backfill.
+ */
+
+/* testCalc1 */
+$a = 667_083 - 11; // Calculation.
+
+/* test Calc2 */
+$a = 6.674_08e3 + 11; // Calculation.
diff --git a/tests/Core/Tokenizer/BackfillNumericSeparatorTest.php b/tests/Core/Tokenizer/BackfillNumericSeparatorTest.php
index b9369e0533..443cc30ef1 100644
--- a/tests/Core/Tokenizer/BackfillNumericSeparatorTest.php
+++ b/tests/Core/Tokenizer/BackfillNumericSeparatorTest.php
@@ -28,8 +28,11 @@ class BackfillNumericSeparatorTest extends AbstractMethodUnitTest
     public function testBackfill($testData)
     {
         $tokens = self::$phpcsFile->getTokens();
-        $number = $this->getTargetToken($testData['marker'], $testData['type']);
-        $this->assertSame($tokens[$number]['content'], $testData['value']);
+        $number = $this->getTargetToken($testData['marker'], [T_LNUMBER, T_DNUMBER]);
+
+        $this->assertSame(constant($testData['type']), $tokens[$number]['code']);
+        $this->assertSame($testData['type'], $tokens[$number]['type']);
+        $this->assertSame($testData['value'], $tokens[$number]['content']);
 
     }//end testBackfill()
 
@@ -43,66 +46,335 @@ public function testBackfill($testData)
      */
     public function dataTestBackfill()
     {
+        $testHexType = 'T_LNUMBER';
+        if (PHP_INT_MAX < 0xCAFEF00D) {
+            $testHexType = 'T_DNUMBER';
+        }
+
+        $testHexMultipleType = 'T_LNUMBER';
+        if (PHP_INT_MAX < 0x42726F776E) {
+            $testHexMultipleType = 'T_DNUMBER';
+        }
+
+        $testIntMoreThanMaxType = 'T_LNUMBER';
+        if (PHP_INT_MAX < 10223372036854775807) {
+            $testIntMoreThanMaxType = 'T_DNUMBER';
+        }
+
         return [
             [
                 [
                     'marker' => '/* testSimpleLNumber */',
-                    'type'   => T_LNUMBER,
+                    'type'   => 'T_LNUMBER',
                     'value'  => '1_000_000_000',
                 ],
             ],
             [
                 [
                     'marker' => '/* testSimpleDNumber */',
-                    'type'   => T_DNUMBER,
+                    'type'   => 'T_DNUMBER',
                     'value'  => '107_925_284.88',
                 ],
             ],
             [
                 [
                     'marker' => '/* testFloat */',
-                    'type'   => T_DNUMBER,
+                    'type'   => 'T_DNUMBER',
                     'value'  => '6.674_083e-11',
                 ],
             ],
             [
                 [
                     'marker' => '/* testFloat2 */',
-                    'type'   => T_DNUMBER,
+                    'type'   => 'T_DNUMBER',
                     'value'  => '6.674_083e+11',
                 ],
             ],
+            [
+                [
+                    'marker' => '/* testFloat3 */',
+                    'type'   => 'T_DNUMBER',
+                    'value'  => '1_2.3_4e1_23',
+                ],
+            ],
             [
                 [
                     'marker' => '/* testHex */',
-                    'type'   => T_LNUMBER,
+                    'type'   => $testHexType,
                     'value'  => '0xCAFE_F00D',
                 ],
             ],
             [
                 [
                     'marker' => '/* testHexMultiple */',
-                    'type'   => T_LNUMBER,
+                    'type'   => $testHexMultipleType,
                     'value'  => '0x42_72_6F_77_6E',
                 ],
             ],
+            [
+                [
+                    'marker' => '/* testHexInt */',
+                    'type'   => 'T_LNUMBER',
+                    'value'  => '0x42_72_6F',
+                ],
+            ],
             [
                 [
                     'marker' => '/* testBinary */',
-                    'type'   => T_LNUMBER,
+                    'type'   => 'T_LNUMBER',
                     'value'  => '0b0101_1111',
                 ],
             ],
             [
                 [
                     'marker' => '/* testOctal */',
-                    'type'   => T_LNUMBER,
+                    'type'   => 'T_LNUMBER',
                     'value'  => '0137_041',
                 ],
             ],
+            [
+                [
+                    'marker' => '/* testIntMoreThanMax */',
+                    'type'   => $testIntMoreThanMaxType,
+                    'value'  => '10_223_372_036_854_775_807',
+                ],
+            ],
         ];
 
     }//end dataTestBackfill()
 
 
+    /**
+     * Test that numbers using numeric seperators which are considered parse errors and/or
+     * which aren't relevant to the backfill, do not incorrectly trigger the backfill anyway.
+     *
+     * @param string $testMarker     The comment which prefaces the target token in the test file.
+     * @param array  $expectedTokens The token type and content of the expected token sequence.
+     *
+     * @dataProvider dataNoBackfill
+     * @covers       PHP_CodeSniffer\Tokenizers\PHP::tokenize
+     *
+     * @return void
+     */
+    public function testNoBackfill($testMarker, $expectedTokens)
+    {
+        $tokens = self::$phpcsFile->getTokens();
+        $number = $this->getTargetToken($testMarker, [T_LNUMBER, T_DNUMBER]);
+
+        foreach ($expectedTokens as $key => $expectedToken) {
+            $i = ($number + $key);
+            $this->assertSame($expectedToken['code'], $tokens[$i]['code']);
+            $this->assertSame($expectedToken['content'], $tokens[$i]['content']);
+        }
+
+    }//end testNoBackfill()
+
+
+    /**
+     * Data provider.
+     *
+     * @see testBackfill()
+     *
+     * @return array
+     */
+    public function dataNoBackfill()
+    {
+        return [
+            [
+                '/* testInvalid1 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '100',
+                    ],
+                    [
+                        'code'    => T_STRING,
+                        'content' => '_',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid2 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '1',
+                    ],
+                    [
+                        'code'    => T_STRING,
+                        'content' => '__1',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid3 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '1',
+                    ],
+                    [
+                        'code'    => T_STRING,
+                        'content' => '_',
+                    ],
+                    [
+                        'code'    => T_DNUMBER,
+                        'content' => '.0',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid4 */',
+                [
+                    [
+                        'code'    => T_DNUMBER,
+                        'content' => '1.',
+                    ],
+                    [
+                        'code'    => T_STRING,
+                        'content' => '_0',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid5 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '0',
+                    ],
+                    [
+                        'code'    => T_STRING,
+                        'content' => 'x_123',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid6 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '0',
+                    ],
+                    [
+                        'code'    => T_STRING,
+                        'content' => 'b_101',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid7 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '1',
+                    ],
+                    [
+                        'code'    => T_STRING,
+                        'content' => '_e2',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid8 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '1',
+                    ],
+                    [
+                        'code'    => T_STRING,
+                        'content' => 'e_2',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid9 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '107_925_284',
+                    ],
+                    [
+                        'code'    => T_WHITESPACE,
+                        'content' => ' ',
+                    ],
+                    [
+                        'code'    => T_DNUMBER,
+                        'content' => '.88',
+                    ],
+                ],
+            ],
+            [
+                '/* testInvalid10 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '107_925_284',
+                    ],
+                    [
+                        'code'    => T_COMMENT,
+                        'content' => '/*comment*/',
+                    ],
+                    [
+                        'code'    => T_DNUMBER,
+                        'content' => '.88',
+                    ],
+                ],
+            ],
+            [
+                '/* testCalc1 */',
+                [
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '667_083',
+                    ],
+                    [
+                        'code'    => T_WHITESPACE,
+                        'content' => ' ',
+                    ],
+                    [
+                        'code'    => T_MINUS,
+                        'content' => '-',
+                    ],
+                    [
+                        'code'    => T_WHITESPACE,
+                        'content' => ' ',
+                    ],
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '11',
+                    ],
+                ],
+            ],
+            [
+                '/* test Calc2 */',
+                [
+                    [
+                        'code'    => T_DNUMBER,
+                        'content' => '6.674_08e3',
+                    ],
+                    [
+                        'code'    => T_WHITESPACE,
+                        'content' => ' ',
+                    ],
+                    [
+                        'code'    => T_PLUS,
+                        'content' => '+',
+                    ],
+                    [
+                        'code'    => T_WHITESPACE,
+                        'content' => ' ',
+                    ],
+                    [
+                        'code'    => T_LNUMBER,
+                        'content' => '11',
+                    ],
+                ],
+            ],
+        ];
+
+    }//end dataNoBackfill()
+
+
 }//end class