The issue: PHP has a new error level E_STRICT, since PHP 5.0.
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Typical E_STRICT error - Usage of outdated is_a() function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();
}
?>