Proof of concept: read content from STDIN; doesn't block if there's nothing there. Works by assuming that content will be available on STDIN immediately, which might not always be the case.

stdin-stream_select.php

<?php

$read = array(STDIN);
$write = null;
$except = null;

if ( stream_select($read, $write, $except, 0) > 0 ) {
	$content = stream_get_contents($read[0]);
} else {
	$content = false;
}

var_dump($content);