You are probably looking to get messages from stderr using proc_open. Something like this:
<?php
$cmd = "/path/to/script arguments here";
$cwd = dirname(__FILE__);
$descriptorspec = array(
0 => array("pipe", "r"), // stdin
1 => array("pipe", "w"), // stdout
2 => array("pipe", "w"), // stderr
);
if ( ($process = proc_open($cmd, $descriptorspec, $pipes, $cwd, null)) !== false )
{
// Standard output
$stdout = stream_get_contents($pipes[1]);
fclose($pipes[1]);
// Errors
$stderr = stream_get_contents($pipes[2]);
fclose($pipes[2]);
proc_close($process);
}
?>