Drush on Ubuntu 16.04 with php5.6 installed fails with Call to undefined function cache_get()

[robbrandt]

I have a relatively new Ubuntu 16.04 server running on Amazon. The server does not allow root login. A minimal Drupal 7 site is installed, as well as Drush 8. I installed Drush 8 by following these steps for global access:

http://docs.drush.org/en/8.x/install/

core-status shows:
php5.6 drush.phar core-status
PHP configuration : /etc/php/5.6/cli/php.ini
PHP OS : Linux
Drush script : /home/ubuntu/drush.phar
Drush version : 8.1.12
Drush temp directory : /tmp
Drush configuration :
Drush alias files :

when logged in as the user account, running a drupal upgrade gets:
PHP Fatal error: Uncaught Error: Call to undefined function cache_get() in /home/crm/public_html/includes/module.inc:754
Stack trace:
#0 /home/crm/public_html/includes/module.inc(954): module_implements('system_theme_in...')
#1 /home/crm/public_html/modules/system/system.module(2511): module_invoke_all('system_theme_in...')
#2 /home/crm/public_html/includes/theme.inc(798): _system_rebuild_theme_data()
#3 /home/crm/public_html/includes/theme.maintenance.inc(57): list_themes()
#4 /home/crm/public_html/includes/bootstrap.inc(2867): _drupal_maintenance_theme()
#5 /home/crm/public_html/includes/errors.inc(179): drupal_maintenance_theme()
#6 /home/crm/public_html/includes/bootstrap.inc(2604): _drupal_log_error(Array, true)
#7 [internal function]: _drupal_exception_handler(Object(Error))
#8 {main}
thrown in /home/crm/public_html/includes/module.inc on line 754
Drush command terminated abnormally due to an unrecoverable error. [error]
Error: Uncaught Error: Call to undefined function cache_get() in
/home/crm/public_html/includes/module.inc:754
Stack trace:
#0 /home/crm/public_html/includes/module.inc(954):
module_implements('system_theme_in...')
#1 /home/crm/public_html/modules/system/system.module(2511):
module_invoke_all('system_theme_in...')
#2 /home/crm/public_html/includes/theme.inc(798):
_system_rebuild_theme_data()
#3 /home/crm/public_html/includes/theme.maintenance.inc(57):
list_themes()
#4 /home/crm/public_html/includes/bootstrap.inc(2867):
_drupal_maintenance_theme()
#5 /home/crm/public_html/includes/errors.inc(179):
drupal_maintenance_theme()
#6 /home/crm/public_html/includes/bootstrap.inc(2604):
_drupal_log_error(Array, true)
#7 [internal function]: _drupal_exception_handler(Object(Error))
#8 {main}
thrown in /home/crm/public_html/includes/module.inc, line 754

This seems to be an error related to using php7 instead of 5.6. I specified 5.6 in the installation.

How do I resolve this?

[robbrandt]

I sudo'd into root, just to see if it's a permission problem. Running as root produces the same error.

[robbrandt]

I did do a backtrace before the get_cache function, and it revealed the issue is "Undefined class constant 'MYSQL_ATTR_SSL_CA'" (full backtrace listed below).

Lots of helpful hints here, but I don't have any of these problems. All Mysql libraries loaded and working.
https://stackoverflow.com/questions/39740933/php-fatal-error-undefined-class-content-drush

#0 module_implements(system_theme_info) called at [/home/crm/domains/testing.crm.botany.org/public_html/includes/module.inc:957]
#1 module_invoke_all(system_theme_info) called at [/home/crm/domains/testing.crm.botany.org/public_html/modules/system/system.module:2511]
#2 _system_rebuild_theme_data() called at [/home/crm/domains/testing.crm.botany.org/public_html/includes/theme.inc:798]
#3 list_themes() called at [/home/crm/domains/testing.crm.botany.org/public_html/includes/theme.maintenance.inc:57]
#4 _drupal_maintenance_theme() called at [/home/crm/domains/testing.crm.botany.org/public_html/includes/bootstrap.inc:2867]
#5 drupal_maintenance_theme() called at [/home/crm/domains/testing.crm.botany.org/public_html/includes/errors.inc:179]
#6 _drupal_log_error(Array ([%type] => Error,[!message] => Undefined class constant 'MYSQL_ATTR_SSL_CA',[%function] => Drush\Sql\Sqlmysql->creds(),[%file] => phar:///usr/bin/drush/lib/Drush/Sql/Sqlmysql.php,[%line] => 56,[severity_level] => 3), 1) called at [/home/crm/domains/testing.crm.botany.org/public_html/includes/bootstrap.inc:2604]
#7 _drupal_exception_handler(Error Object ([*message] => Undefined class constant 'MYSQL_ATTR_SSL_CA',[Errorstring] => ,[*code] => 0,[*file] => phar:///usr/bin/drush/lib/Drush/Sql/Sqlmysql.php,[*line] => 56,[Errortrace] => Array ([0] => Array ([file] => phar:///usr/bin/drush/lib/Drush/Sql/SqlBase.php,[line] => 164,[function] => creds,[class] => Drush\Sql\Sqlmysql,[type] => ->,[args] => Array ()),[1] => Array ([file] => phar:///usr/bin/drush/includes/environment.inc,[line] => 393,[function] => query,[class] => Drush\Sql\SqlBase,[type] => ->,[args] => Array ([0] => SELECT 1;)),[2] => Array ([file] => phar:///usr/bin/drush/lib/Drush/Boot/DrupalBoot.php,[line] => 462,[function] => drush_valid_db_credentials,[args] => Array ()),[3] => Array ([file] => phar:///usr/bin/drush/lib/Drush/Boot/DrupalBoot7.php,[line] => 70,[function] => bootstrap_drupal_database_validate,[class] => Drush\Boot\DrupalBoot,[type] => ->,[args] => Array ()),[4] => Array ([file] => phar:///usr/bin/drush/includes/bootstrap.inc,[line] => 430,[function] => bootstrap_drupal_database_validate,[class] => Drush\Boot\DrupalBoot7,[type] => ->,[args] => Array ()),[5] => Array ([file] => phar:///usr/bin/drush/includes/bootstrap.inc,[line] => 471,[function] => drush_bootstrap_validate,[args] => Array ([0] => 4)),[6] => Array ([file] => phar:///usr/bin/drush/lib/Drush/Boot/BaseBoot.php,[line] => 59,[function] => drush_bootstrap_to_phase,[args] => Array ([0] => 6)),[7] => Array ([file] => phar:///usr/bin/drush/includes/preflight.inc,[line] => 66,[function] => bootstrap_and_dispatch,[class] => Drush\Boot\BaseBoot,[type] => ->,[args] => Array ()),[8] => Array ([file] => phar:///usr/bin/drush/includes/startup.inc,[line] => 458,[function] => drush_main,[args] => Array ()),[9] => Array ([file] => phar:///usr/bin/drush/includes/startup.inc,[line] => 365,[function] => drush_run_main,[args] => Array ([0] => ,[1] => /,[2] => Phar detected. Proceeding to drush_main().)),[10] => Array ([file] => phar:///usr/bin/drush/drush,[line] => 114,[function] => drush_startup,[args] => Array ([0] => Array ([0] => /usr/bin/drush,[1] => pm-update,[2] => drupal))),[11] => Array ([file] => /usr/bin/drush,[line] => 10,[args] => Array ([0] => phar:///usr/bin/drush/drush),[function] => require)),[Errorprevious] => ))

[weitzman]

Not a drush issue