Quick and dirty was to resize an image canvas, center original and fill white space

Resize Image Canvas

<?php

function processImage($imgname)
{
    // Load original image
    $original = @imagecreatefromjpeg($imgname);

    if (!$original)
        die('Bad Image: ' . $imgname);

    // Get size of original image
    $size = getimagesize($imgname);

    // Extract width and height
    $w = $size[0];
    $h = $size[1];

    // Set equal width and height along with an offset if one was different
    if ($w > $h)
    {
        $width = $w;
        $height = $w;
        $pos_x = 0;
        $pos_y = ($width - $h) / 2;
        $pos_y = $pos_y - ($pos_y * 2);
    }
    else if ($h > $w)
    {
        $width = $h;
        $height = $h;
        $pos_x = ($height - $w) / 2;
        $pos_x = $pos_x - ($pos_x * 2);
        $pos_y = 0;
    }
    else
    {
        $width = $w;
        $height = $h;
        $pos_x = 0;
        $pos_y = 0;
    }

    // Create a new image with white background
    $canvas = imagecreatetruecolor($width, $height);
    $white = imagecolorallocate($canvas, 255, 255, 255);
    imagefilledrectangle($canvas, 0, 0, $width, $height, $white);

    // Copy original image into new image at offsets set above
    imagecopy($canvas, $original, 0, 0, $pos_x, $pos_y, $width, $height);

    return $canvas;
}

// Array of images to be processed, could use readdir to open all files in a directory
$images = array('image.jpg');

foreach ($images as $image)
{
    $img = processImage($image);

    // Save image over original
    imagejpeg($img, $image);
    imagedestroy($img);
}