Drupal 7 stopped working 500 internal Server error For some reasons, my site stopped responding today, Error 


  The www.mysite.com page isn’t working
  
  www.mysite.com is currently unable to handle this request. HTTP ERROR
  500


I understand there are many reasons for 500 error and it hard to know what exactly happend, I had a look into server log and notice this error 


  [Wed May 11 10:31:27 2016] [warn] [client x.y.z] mod_fcgid: stderr:
  PHP Parse error: syntax error, unexpected 'return' (T_RETURN) in
  /var/www/vhosts/mysite.com/httpdocs/pro/includes/common.inc on line
  5106.


Looking into common.inc

function drupal_json_encode($var) {
  // The PHP version cannot change within a request.
  static $php530;

  if (!isset($php530)) {
    $php530 = version_compare(PHP_VERSION, '5.3.0', '&gt;=');
  }

  if ($php530) {
    // Encode &lt;, &gt;, ', &amp;, and " using the json_encode() options parameter.
    return json_encode($var, JSON_HEX_TAG | JSON_HEX_APOS | JSON_HEX_AMP | JSON_HEX_QUOT);
  }

  // json_encode() escapes &lt;, &gt;, ', &amp;, and " using its options parameter, but
  // does not support this parameter prior to PHP 5.3.0.  Use a helper instead.
  include_once DRUPAL_ROOT . '/includes/json-encode.inc';
  return drupal_json_encode_helper($var);
}


Line 5106 =   return json_encode($var, JSON_HEX_TAG | JSON_HEX_APOS | JSON_HEX_AMP | JSON_HEX_QUOT);
For some reason under PHP selector in Plesk dom and gd was disabled. after enabling those two the site return back to normal.