<?php

$imageWidth = 1000;
$imageHeight = 500;
$ar = 1.25;

if ($ar < 1) { // "tall" crop
    $cropWidth = min($imageHeight * $ar, $imageWidth);
    $cropHeight = $cropWidth / $ar;
}
else { // "wide" or square crop
    $cropHeight = min($imageWidth / $ar, $imageHeight);
    $cropWidth = $cropHeight * $ar;
}

echo "Cropped: $cropWidth x $cropHeight";