The issue: PHP has a new error level E_STRICT, since PHP 5.0.
E_STRICT forces you to code clean PHP 5 code
Current E_STRICT will most probably become E_FATAL in PHP 6.0
The resolution: Switch E_STRICT on and check your code
error_reporting=E_ALL | E_STRICT
Typical E_STRICT error – Usage of outdated is_a() function
<?php if ( is_a( $object, 'ClassName' ) ) { $object->foo(); } ?>
Should be now
<?php if ( $object instanceof ClassName ) { $object->foo(); } ?>