3 ways to test if a file is an image

imageTest.php

<?php

/**
 * Test if given file is a web image (gif/jpg/png)
 * 
 * Requires PHP 5.3 or newer (builtin) or the Fileinfo PECL extension.
 * 
 * @param  string $file Path to image
 * @return boolean      True if web image, or false if unreadable or not image
 */ 
function finfo_is_image($file)
{
    if (!is_readable($file)) {
        return false;
    }

    $finfo = new Finfo(FILEINFO_MIME_TYPE);
    $type  = $finfo->file($file);
        
    switch($type) {
        case 'image/jpeg':
        case 'image/png':
        case 'image/gif':
            return true;
        default:
            return false;
    }
}

/**
 * Test if given file is a web image (gif/jpg/png)
 * 
 * Uses the getimagesize() built-in function. Don't use it on a URL because it
 * will download the whole image before checking.
 * 
 * @param  string $file Path to image
 * @return mixed        Returns "gif", "jpg", "png", or false if unreadable or not image
 */
function getimagesize_is_image($file)
{
    if (!(is_file($file) && is_readable($file))) {
        return false;
    }

    $info = getimagesize($file);
    
    if (!is_array($info)) {
        return false;
    }
    
    switch($info[2]) {
        case IMAGETYPE_JPEG:
        case IMAGETYPE_PNG:
        case IMAGETYPE_GIF:
            return true;
        default:
            return false;
    }
}

/**
 * Test if given file is a web image (gif/jpg/png)
 * 
 * More URL-safe than getimagesize method & works on any PHP version.
 * 
 * @param  string $image_path   Path to image
 * @return boolean              True if web image, or false if unreadable or not image
 */ 
function fopen_is_image($image_path)
{
    if (!(is_file($image_path) && is_readable($image_path))) {
        return false;
    }
    
    if (!$f = fopen($image_path, 'rb'))
    {
        return false;
    }

    $data = fread($f, 8);
    fclose($f);

    // signature checking
    $unpacked = unpack("H12", $data);
    if (array_pop($unpacked) == '474946383961' || array_pop($unpacked) == '474946383761') return true;
    $unpacked = unpack("H4", $data);
    if (array_pop($unpacked) == 'ffd8') return true;
    $unpacked = unpack("H16", $data);
    if (array_pop($unpacked) == '89504e470d0a1a0a') return true;

    return false;
}



// Tests:
$tests = array(
    '/Users/matt/Downloads/cougar_6.jpg'            => true,
    '/Users/matt/Downloads/IMG_16102012_161427.png' => true,
    '/Users/matt/Downloads/691493.gif'              => true,
    '/Users/matt/Downloads/Stranger-in-moscow.mp3'  => false,
    '/filenotfound'                                 => false,
    '/tmp/unreadable'                               => false,
    '/tmp'                                          => false,
);

echo "finfo_is_image():".PHP_EOL;
foreach ($tests as $file => $expected) {
    echo $file . '...';
    if ($expected) {
        echo assert(finfo_is_image($file)) ? 'OK' : 'FAILED';
    } else {
        echo assert(!finfo_is_image($file)) ? 'OK' : 'FAILED';
    }
    echo PHP_EOL;
}

echo PHP_EOL."getimagesize_is_image():".PHP_EOL;
foreach ($tests as $file => $expected) {
    echo $file . '...';
    if ($expected) {
        echo assert(getimagesize_is_image($file)) ? 'OK' : 'FAILED';
    } else {
        echo assert(!getimagesize_is_image($file)) ? 'OK' : 'FAILED';
    }
    echo PHP_EOL;
}

echo PHP_EOL."fopen_is_image():".PHP_EOL;
foreach ($tests as $file => $expected) {
    echo $file . '...';
    if ($expected) {
        echo assert(fopen_is_image($file)) ? 'OK' : 'FAILED';
    } else {
        echo assert(!fopen_is_image($file)) ? 'OK' : 'FAILED';
    }
    echo PHP_EOL;
}