PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to all

I am using wordpress.

I have modified in /etc/php5/cli/php.ini memory_limit=512M,
also in wp-config.php, added define( ‘WP_MEMORY_LIMIT’, ‘512M’ );
and in /etc/php5/apache2/php.ini memory_limit = 512M,
Not reflecting the new memory 512M.
when I am trying to access wordpress admin panel, after login showing only white page and showing following error in error log

PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 4194304 bytes) in /var/www/testing/wp-includes/option.php on line 253
PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 4194304 bytes) in /var/www/testing/wp-includes/option.php on line 253
PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 4194304 bytes) in /var/www/testing/wp-includes/option.php on line 253
PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 786432 bytes) in /var/www/testing/wp-includes/SimplePie/Item.php on line 2736