Run WP-CLI from PHP Script

Question

I am currently trying to run a wp cli command from a php script. For those unfamiliar with WP-CLI its a cool command line interface for Wordpress. Below is my following script:

<?php
// lets create a basic test
$cmd = '/usr/local/bin/wp core download';

$descriptorspec = array(
   0 => array("pipe", "r"),  // stdin is a pipe that the child will read from
   1 => array("pipe", "w"),  // stdout is a pipe that the child will write to
   2 => array("file", "path-to-error-log/error-output.txt", "a") // stderr is a file to   write to
);

$process = proc_open($cmd, $descriptorspec, $pipes);

if (is_resource($process)) {
   // $pipes now looks like this:
   // 0 => writeable handle connected to child stdin
   // 1 => readable handle connected to child stdout

   $output = stream_get_contents($pipes[1]);
   fclose($pipes[1]);

   // It is important that you close any pipes before calling
   // proc_close in order to avoid a deadlock
   $return_value = proc_close($process);

   echo $output;

}


?>

Inside error-output.txt i receive the following error:

sh: /usr/local/bin/wp: Permission denied

From what I can gather this is a permission issue, where apache (the current user executing my php script) is unable to for some reason execute the wp bin file.

Out of curiosity I have done the following:

1. Changed the command i want to run to wp --info which outputs information about wp-cli and does not require writing to another directory, i assumed this could have been an issue but i was proved wrong since i still get the permission denied error.
2. I tried to move the wp bin to usr/bin instead. No effect here either.
3. Out of desperation I even tried to give wp 777 permissions and apache was still unable to execute.

I realise this is more of a server setup/permission issue (possibly) but I felt this would be the best place to ask.