Php errors not logging for wordpress plugins

Solution:

As the Codex says:

WP_DEBUG_LOG is companion to WP_DEBUG that causes all errors to also be saved to a debug.log log file inside the /wp-content/ directory. This is useful if you want to review all notices later or need to view notices generated off-screen (e.g. during an AJAX request or wp-cron run).

Edit your wp-config.php file and add the following definition (if not already present):

define('WP_DEBUG_LOG', true);

This will make WordPress log the errors to /wp-content/debug.log.

However, if you want to change the location of the debug.log file, you can use the following work-around:

if (defined('WP_DEBUG_LOG') && WP_DEBUG_LOG) {
    ini_set( 'error_log', WP_CONTENT_DIR.'/foo/debug.log' ); //modify accordingly
}