diff --git a/src/Calculator.php b/src/Calculator.php
index 6831cf15..99909bed 100644
--- a/src/Calculator.php
+++ b/src/Calculator.php
@@ -21,8 +21,8 @@ public static function supported();
     /**
      * Compare a to b
      *
-     * @param int|string $a
-     * @param int|string $b
+     * @param string $a
+     * @param string $b
      * @return int
      */
     public function compare($a, $b);
@@ -30,70 +30,70 @@ public function compare($a, $b);
     /**
      * Add added to amount
      *
-     * @param int|string $amount
-     * @param int|string $addend
-     * @return int|string
+     * @param string $amount
+     * @param string $addend
+     * @return string
      */
     public function add($amount, $addend);
 
     /**
      * Subtract subtrahend from amount
-     * @param int|string $amount
-     * @param int|string $subtrahend
-     * @return int|string
+     * @param string $amount
+     * @param string $subtrahend
+     * @return string
      */
     public function subtract($amount, $subtrahend);
 
     /**
      * Multiply amount with multiplier
      *
-     * @param int|string $amount
-     * @param int|float $multiplier
-     * @return int|string
+     * @param string $amount
+     * @param int|float|string $multiplier
+     * @return string
      */
     public function multiply($amount, $multiplier);
 
     /**
      * Divide amount with divisor
      *
-     * @param int|string $amount
-     * @param int|float $divisor
-     * @return int|string
+     * @param string $amount
+     * @param int|float|string $divisor
+     * @return string
      */
     public function divide($amount, $divisor);
 
     /**
      * Round number to following integer
      *
-     * @param int|string|float $number
-     * @return int|string
+     * @param string $number
+     * @return string
      */
     public function ceil($number);
 
     /**
      * Round number to preceding integer
      *
-     * @param int|string|float $number
-     * @return int|string
+     * @param string $number
+     * @return string
      */
     public function floor($number);
 
     /**
      * Round number, use rounding mode for tie-breaker
      *
-     * @param int|string|float $number
-     * @param int|string $roundingMode
-     * @return int|string
+     * @param string $number
+     * @param int $roundingMode
+     * @return string
      */
     public function round($number, $roundingMode);
 
     /**
      * Share amount among ratio / total portions
      *
-     * @param int|string $amount
+     * @param string $amount
      * @param int|float $ratio
      * @param int|float $total
-     * @return int|string
+     * @return string
      */
     public function share($amount, $ratio, $total);
 }
diff --git a/src/Calculator/PhpCalculator.php b/src/Calculator/PhpCalculator.php
index b0687ad3..0e43475a 100644
--- a/src/Calculator/PhpCalculator.php
+++ b/src/Calculator/PhpCalculator.php
@@ -34,7 +34,7 @@ public function add($amount, $addend)
 
         $this->assertInteger($result);
 
-        return $result;
+        return (string) $result;
     }
 
     /**
@@ -46,7 +46,7 @@ public function subtract($amount, $subtrahend)
 
         $this->assertInteger($result);
 
-        return $result;
+        return (string) $result;
     }
 
     /**
@@ -58,7 +58,7 @@ public function multiply($amount, $multiplier)
 
         $this->assertIntegerBounds($result);
 
-        return $result;
+        return (string) $result;
     }
 
     /**
@@ -70,7 +70,7 @@ public function divide($amount, $divisor)
 
         $this->assertIntegerBounds($result);
 
-        return $result;
+        return (string) $result;
     }
 
     /**
@@ -134,7 +134,7 @@ private function castInteger($amount)
     {
         $this->assertIntegerBounds($amount);
 
-        return intval($amount);
+        return (string) intval($amount);
     }
 
     /**
@@ -144,7 +144,7 @@ private function castInteger($amount)
      */
     private function assertInteger($amount)
     {
-        if (!is_int($amount)) {
+        if (filter_var($amount, FILTER_VALIDATE_INT) === false) {
             throw new \UnexpectedValueException('The result of arithmetic operation is not an integer');
         }
     }
diff --git a/src/Money.php b/src/Money.php
index 6e7f5c5f..e98a8993 100644
--- a/src/Money.php
+++ b/src/Money.php
@@ -47,7 +47,7 @@ final class Money implements JsonSerializable
     ];
 
     /**
-     * @param int      $amount   Amount, expressed in the smallest units of $currency (eg cents)
+     * @param int|string $amount   Amount, expressed in the smallest units of $currency (eg cents)
      * @param Currency $currency
      *
      * @throws InvalidArgumentException If amount is not integer
@@ -58,7 +58,7 @@ public function __construct($amount, Currency $currency)
             throw new InvalidArgumentException('Amount must be an integer');
         }
 
-        $this->amount = $amount;
+        $this->amount = (string) $amount;
         $this->currency = $currency;
     }
 
@@ -84,7 +84,7 @@ public static function __callStatic($method, $arguments)
     /**
      * Returns a new Money instance based on the current one using the Currency.
      *
-     * @param int $amount
+     * @param int|string $amount
      *
      * @return Money
      *
@@ -196,7 +196,7 @@ public function lessThanOrEqual(Money $other)
     /**
      * Returns the value represented by this object.
      *
-     * @return int
+     * @return string
      */
     public function getAmount()
     {
@@ -282,7 +282,7 @@ private function assertRoundingMode($roundingMode)
      * Returns a new Money object that represents
      * the multiplied value by the given factor.
      *
-     * @param numeric $multiplier
+     * @param float|int|string $multiplier
      * @param int     $roundingMode
      *
      * @return Money
@@ -316,7 +316,7 @@ public function convert(Currency $targetCurrency, $conversionRate, $roundingMode
      * Returns a new Money object that represents
      * the divided value by the given factor.
      *
-     * @param numeric $divisor
+     * @param float|int|string $divisor
      * @param int     $roundingMode
      *
      * @return Money
@@ -384,7 +384,7 @@ public function allocateTo($n)
      * @param int|float $amount
      * @param $rounding_mode
      *
-     * @return int
+     * @return string
      */
     private function round($amount, $rounding_mode)
     {
@@ -434,7 +434,7 @@ public function isNegative()
      *
      * @param string $string
      *
-     * @return int
+     * @return string
      *
      * @throws InvalidArgumentException If $string cannot be parsed
      */
@@ -455,7 +455,7 @@ public static function stringToUnits($string)
         $units .= isset($matches['decimal1']) ? $matches['decimal1'] : '0';
         $units .= isset($matches['decimal2']) ? $matches['decimal2'] : '0';
 
-        return (int) $units;
+        return (string) $units;
     }
 
     /**
diff --git a/tests/MoneyTest.php b/tests/MoneyTest.php
index d8bb757d..debd467f 100644
--- a/tests/MoneyTest.php
+++ b/tests/MoneyTest.php
@@ -66,10 +66,12 @@ public function testEquality()
         $m2 = new Money(100, new Currency('EUR'));
         $m3 = new Money(100, new Currency('USD'));
         $m4 = new Money(50, new Currency('EUR'));
+        $m5 = new Money('100', new Currency('EUR'));
 
         $this->assertTrue($m1->equals($m2));
         $this->assertFalse($m1->equals($m3));
         $this->assertFalse($m1->equals($m4));
+        $this->assertTrue($m1->equals($m5));
     }
 
     public function testAddition()
@@ -296,8 +298,11 @@ public function testAllocationToInvalidTargets()
     public function testComparators()
     {
         $this->assertTrue(Money::EUR(0)->isZero());
+        $this->assertTrue(Money::EUR('0')->isZero());
         $this->assertTrue(Money::EUR(-1)->isNegative());
+        $this->assertTrue(Money::EUR('-1')->isNegative());
         $this->assertTrue(Money::EUR(1)->isPositive());
+        $this->assertTrue(Money::EUR('1')->isPositive());
         $this->assertFalse(Money::EUR(1)->isZero());
         $this->assertFalse(Money::EUR(1)->isNegative());
         $this->assertFalse(Money::EUR(-1)->isPositive());
@@ -347,7 +352,7 @@ public function testCannotConvertStringToUnits()
     public function testJsonEncoding()
     {
         $this->assertEquals(
-            '{"amount":350,"currency":"USD"}',
+            '{"amount":"350","currency":"USD"}',
             json_encode(Money::USD(350))
         );
     }
diff --git a/tests/PhpCalculatorTest.php b/tests/PhpCalculatorTest.php
index abf190ef..02c152c9 100644
--- a/tests/PhpCalculatorTest.php
+++ b/tests/PhpCalculatorTest.php
@@ -10,68 +10,68 @@ class PhpCalculatorTest extends \PHPUnit_Framework_TestCase
     public function testCompare()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals(1, $calculator->compare(2, 1));
-        $this->assertEquals(-1, $calculator->compare(1, 2));
-        $this->assertEquals(0, $calculator->compare(1, 1));
+        $this->assertSame(1, $calculator->compare(2, 1));
+        $this->assertSame(-1, $calculator->compare(1, 2));
+        $this->assertSame(0, $calculator->compare(1, 1));
     }
 
     public function testAdd()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals(2, $calculator->add(1, 1));
+        $this->assertSame('2', $calculator->add(1, 1));
     }
 
     public function testSubtract()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals(1, $calculator->subtract(2, 1));
+        $this->assertSame('1', $calculator->subtract(2, 1));
     }
 
     public function testMultiply()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals('1.5', $calculator->multiply(1, 1.5));
+        $this->assertSame('1.5', $calculator->multiply(1, 1.5));
     }
 
     public function testDivide()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals('1.5', $calculator->divide(3, 2));
+        $this->assertSame('1.5', $calculator->divide(3, 2));
     }
 
     public function testCeil()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals('2', $calculator->ceil(1.2));
+        $this->assertSame('2', $calculator->ceil(1.2));
     }
 
     public function testFloor()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals('2', $calculator->floor(2.7));
+        $this->assertSame('2', $calculator->floor(2.7));
     }
 
     public function testRound()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals('3', $calculator->round(2.6, Money::ROUND_HALF_EVEN));
-        $this->assertEquals('2', $calculator->round(2.5, Money::ROUND_HALF_EVEN));
-        $this->assertEquals('2', $calculator->round(2.1, Money::ROUND_HALF_ODD));
-        $this->assertEquals('3', $calculator->round(2.5, Money::ROUND_HALF_ODD));
-        $this->assertEquals('2', $calculator->round(2.5, Money::ROUND_HALF_DOWN));
-        $this->assertEquals('3', $calculator->round(2.6, Money::ROUND_HALF_DOWN));
-        $this->assertEquals('2', $calculator->round(2.2, Money::ROUND_HALF_UP));
-        $this->assertEquals('3', $calculator->round(2.5, Money::ROUND_HALF_UP));
-        $this->assertEquals('2', $calculator->round(2, Money::ROUND_HALF_UP));
-        $this->assertEquals('2', $calculator->round(2, Money::ROUND_HALF_DOWN));
-        $this->assertEquals('2', $calculator->round(2, Money::ROUND_HALF_EVEN));
-        $this->assertEquals('2', $calculator->round(2, Money::ROUND_HALF_ODD));
+        $this->assertSame('3', $calculator->round(2.6, Money::ROUND_HALF_EVEN));
+        $this->assertSame('2', $calculator->round(2.5, Money::ROUND_HALF_EVEN));
+        $this->assertSame('2', $calculator->round(2.1, Money::ROUND_HALF_ODD));
+        $this->assertSame('3', $calculator->round(2.5, Money::ROUND_HALF_ODD));
+        $this->assertSame('2', $calculator->round(2.5, Money::ROUND_HALF_DOWN));
+        $this->assertSame('3', $calculator->round(2.6, Money::ROUND_HALF_DOWN));
+        $this->assertSame('2', $calculator->round(2.2, Money::ROUND_HALF_UP));
+        $this->assertSame('3', $calculator->round(2.5, Money::ROUND_HALF_UP));
+        $this->assertSame('2', $calculator->round(2, Money::ROUND_HALF_UP));
+        $this->assertSame('2', $calculator->round(2, Money::ROUND_HALF_DOWN));
+        $this->assertSame('2', $calculator->round(2, Money::ROUND_HALF_EVEN));
+        $this->assertSame('2', $calculator->round(2, Money::ROUND_HALF_ODD));
     }
 
     public function testShare()
     {
         $calculator = new PhpCalculator();
-        $this->assertEquals('5', $calculator->share(10, 2, 4));
+        $this->assertSame('5', $calculator->share(10, 2, 4));
     }
 
     /**