This would appear to be the equivalent of +repage in ImageMagick.
<?php
$ImagickObj = new Imagick($sourceImagePath);
$ImagickObj->cropImage($_POST['w'], $_POST['h'], $_POST['x'], $_POST['y']);
$ImagickObj->setImagePage($_POST['w'], $_POST['h'], 0, 0);
$ImagickObj->writeImage($cropFilePath);
?>