title: php shell_exec return null
body: <p>I want run <code>php ../cat1/index.php gr_s2/3/gr-n40 1200</code> command by php.
  The shell_exec return NULL as result but when I try that command on cmd, output was shown correctly. </p>
  
  <p>What happen in php and shell_exec ?!</p>
  
  <p>Note: The command with different parameters (Like: <code>php ../cat1/index.php gr_s2/3/gr-n40 800</code>) works correctly in both (php and cmd).</p>