php_value memory_limit 256M
php_value register_globals off
php_value max_execution_time 600
php_value upload_max_filesize 64M
php_value post_max_size 64M

define('WP_MEMORY_LIMIT', '256M');
define( 'WP_MAX_MEMORY_LIMIT', '256M' );

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 19628820 bytes) in /path/to/wordpress/wp-includes/wp-db.php on line 2888