Blur image dynamically

blur.php

<?php 

/**
* Strong Blur
*
* @param resource $gdImageResource 
* @param int $blurFactor optional 
*  This is the strength of the blur
*  0 = no blur, 3 = default, anything over 5 is extremely blurred
* @return GD image resource
* @author Martijn Frazer, idea based on http://stackoverflow.com/a/20264482
*/
function blur($gdImageResource, $blurFactor = 3)
{
    $blurFactor     = round($blurFactor);  
    $originalWidth  = imagesx($gdImageResource);
    $originalHeight = imagesy($gdImageResource);
    $smallestWidth  = ceil($originalWidth * pow(0.5, $blurFactor));
    $smallestHeight = ceil($originalHeight * pow(0.5, $blurFactor));
    $prevImage      = $gdImageResource;
    $prevWidth      = $originalWidth;
    $prevHeight     = $originalHeight;

    for($i = 0; $i < $blurFactor; $i += 1)
    {
        $nextWidth = $smallestWidth * pow(2, $i);
        $nextHeight = $smallestHeight * pow(2, $i);
        $nextImage = imagecreatetruecolor($nextWidth, $nextHeight);
        imagecopyresized($nextImage, $prevImage, 0, 0, 0, 0, $nextWidth, $nextHeight, $prevWidth, $prevHeight);
        imagefilter($nextImage, IMG_FILTER_GAUSSIAN_BLUR);
        $prevImage  = $nextImage;
        $prevWidth  = $nextWidth;
        $prevHeight = $nextHeight;
    }
    imagecopyresized($gdImageResource, $nextImage, 0, 0, 0, 0, $originalWidth, $originalHeight, $nextWidth, $nextHeight);
    imagefilter($gdImageResource, IMG_FILTER_GAUSSIAN_BLUR);
    imagedestroy($prevImage);

    return $gdImageResource;
}

$im = imagecreatefromjpeg($_GET['href']);
header('Content-Type: image/jpeg');
blur($im, 4);
imagejpeg($im);
imagedestroy($im);

Example usage:

http://localhost/blur.php?href=https://placebear.com/400/200