Generate GIF, JPG, and PNG thumbnails on the fly

If you need to quickly generate thumbnails with PHP you can use this code.

<?php

// create and return a thumbnail version of an image

// set up height and width of the created thmbnail
if (isset($_GET['width'])) {
    $newWidth = $_GET['width'];
} else {
    $newWidth = 100;
}

if (isset($_GET['height'])) {
    $newHeight = $_GET['height'];
} else {
    $newHeight = 100;
}

if (isset($_GET['quality'])) {
    $quality = intval($_GET['quality']);
    if ($quality > 100) $quality = 100;
    if ($quality < 1) $quality = 1;
} else {
    $quality = 80;
}

// read in the passed image
if(substr($src, strlen($src)-3,3)=="gif" || substr($src, strlen($src)-3,3)=="GIF"){
    $im = imagecreatefromgif("$src"); // Attempt to open
}
if(substr($src, strlen($src)-3,3)=="jpg" || substr($src, strlen($src)-3,3)=="JPG" || substr($src, strlen($src)-3,3)=="peg" || substr($src, strlen($src)-3,3)=="PEG"){
    $im = imagecreatefromjpeg ("$src"); // Attempt to open
}
if(substr($src, strlen($src)-3,3)=="png" || substr($src, strlen($src)-3,3)=="PNG"){
    $im = imagecreatefrompng ("$src"); // Attempt to open
}

if (isset($_GET['mode'])) {
    $mode = $_GET['mode'];
    // only allow two modes (fixed and proportional)
    // and set the default to proportional if an unknown mode is set or no incoming
    // setting is detected
    if ($mode != 'proportional' && $mode != 'fixed') {
        $mode = 'proportional';
    }
} else {
    $mode = 'proportional';
}

if ($mode == 'proportional') {
    // Create thumbnails constraining their MAXIMUM size and retaining proportions

    //obtain the original image Height and Width
    $srcWidth = imagesx($im);
    $srcHeight = imagesy($im);

    // start by using width for calculations
    $percentage = (double)$newWidth / $srcWidth;
    $destHeight = round($srcHeight * $percentage);
    $destWidth = round($srcWidth * $percentage);

    if($destHeight > $newHeight){
        // if the width produces a height bigger than we want, calculate based on height
        $percentage =(double)$newHeight / $srcHeight;
        $destHeight = round($srcHeight * $percentage);
        $destWidth = round($srcWidth * $percentage);
    }
} else {
    $destHeight = $newHeight;
    $destWidth  = $newWidth;
}

// create a blank image of the required size
$new = imagecreatetruecolor ($destWidth, $destHeight);

// copy a resied version of the original to the new image
imagecopyresampled($new,$im,0,0,0,0,$destWidth,$destHeight, imagesx($im),imagesy($im));

// output header and file
header ("content-type: image/jpeg");
imagejpeg ($new, NULL, $quality);

// clear the image buffers
imagedestroy ($im);
imagedestroy ($new);
?>

Save it to a file named thumbnail.php (or whatever you like) and use it like this:

// Absolute path to image file
$src     = $_SERVER['DOCUMENT_ROOT'] . 'image.jpg';
$width   = 50;
$height  = 50;
$quality = 100;

$params = "src={$src}&width={$width}&height={$height}&quality={$quality}";
echo '<img src="thumbnail.php?' . $params . '" alt=""/>';

// Or you can omit some params and use the defaults
$params = "src={$src}";
echo '<img src="thumbnail.php?' . $params . '" alt=""/>';

Note that this just opens whatever file you pass to $src (which is a very bad idea). To prevent this security flaw you could hardcode the path inside the thumbnail.php file and just pass a filename via $src.