Convert PNG and JPEG to WebP with a PHP script

For a handful of files, use the image converter on this site — it runs in your browser, nothing is uploaded, and there is no script to deploy. The rest of this page is for a folder full of them on a server.

<?php
if (!extension_loaded('gd')) {
    exit("The GD extension is not enabled. Enable it in php.ini.\n");
}

$directory = '.';
$quality   = 82;   // see the note below -- 100 is not what you want
$converted = $skipped = 0;

foreach (scandir($directory) as $file) {
    $extension = strtolower(pathinfo($file, PATHINFO_EXTENSION));
    if (!in_array($extension, ['jpg', 'jpeg', 'png'], true)) {
        continue;
    }

    $output = pathinfo($file, PATHINFO_FILENAME) . '.webp';
    if (file_exists($output)) {
        continue;
    }

    if ($extension === 'png') {
        $image = imagecreatefrompng($file);
        imagepalettetotruecolor($image);
        imagealphablending($image, true);
        imagesavealpha($image, true);
    } else {
        $image = imagecreatefromjpeg($file);
    }

    if ($image === false) {
        continue;
    }

    imagewebp($image, $output, $quality);
    imagedestroy($image);

    // KEEP THE SMALLER FILE. WebP does not win on every image.
    if (filesize($output) >= filesize($file)) {
        unlink($output);
        $skipped++;
        echo "kept original: {$file}\n";
    } else {
        $converted++;
        $saved = round((1 - filesize($output) / filesize($file)) * 100);
        echo "converted {$file} -> {$output} ({$saved}% smaller)\n";
    }
}

echo "\n{$converted} converted, {$skipped} left alone.\n";

Run it from the folder you want converted. It writes alongside the originals and deletes nothing you had before.

Quality 100 is the wrong setting, and it is the common mistake

PHP's manual describes the parameter as ranging from "0 (worst quality, smaller file) to 100 (best quality, biggest file)", and says that passing -1 uses a default of 80.

That default is the useful hint. At 100 the encoder is told to preserve everything it can, and the result is frequently larger than the JPEG you started with — which defeats the entire purpose. Somewhere in the high 70s to mid 80s is where WebP earns its reputation: visually indistinguishable on photographs, and meaningfully smaller.

82 is a reasonable starting point. Look at the output on a few of your own images before running it over a thousand.

Why the size check matters

WebP is not smaller than everything. It reliably beats JPEG on photographs and PNG on flat graphics, and it loses on:

• an already-optimized PNG or JPEG, where the previous encoder has already done this work;
• very small images, where the container overhead is a large fraction of the file;
• screenshots of large flat areas, which PNG compresses extremely well.

A conversion loop with no size check will happily make those pages heavier while reporting success. Comparing filesize() and deleting the loser costs one line and removes the whole question.

Converting is only half of it

A .webp file next to a .jpg changes nothing until something serves it. You need either markup that offers both:

<picture>
  <source srcset="photo.webp" type="image/webp">
  <img src="photo.jpg" alt="..." width="1200" height="800">
</picture>

...or a build step that emits WebP and rewrites the references for you, which is what most static site generators do. Converting the folder and leaving the HTML pointing at the old files is a common way to end up with twice the images and none of the benefit.