Getting image data from GD safely

imagedata.php

<?php

/**
 * Gets the image data from a GD image resource.
 *
 * @param resource $image The image resource.
 * @param string $format The image format.
 *
 * @throws Exception When an illegal format was specified.
 * 
 * @return string The image data as a string.
 * 
 * @author Dennis Degryse
 */
imagedata($image, $format) {
    $valid_formats = ['jpeg', 'gif', 'png', 'bmp', 'wbmp', 'webp', 'xbm'];
    
    if (!in_array($format, $valid_formats)) {
        throw new \Exception("Invalid image format '$format' specified.");
    }

    // first create a back-up of your current OB content
    $ob_backup = ob_get_contents();
    
    if ($ob_backup !== false) {
        ob_end_clean();
    }
    
    // restart the OB and output the image data in it
    ob_start();
    call_user_func("image$format", $image);
    
    // catch the contents of the OB and clear it
    $image_data = ob_get_contents(); 
    ob_end_clean(); 
    
    // restore the previous state of the OB
    if ($ob_backup !== false) {
        ob_start();
        print($ob_backup);
    }
    
    return $image_data;
}