docker-php-extension-installer/scripts/tests/gd.php

<?php

$imageWidth = 8;
$imageHeight = 16;
$image = imagecreatetruecolor($imageWidth, $imageHeight);
$formats = [
    'gd2',
    'gif',
    'jpeg',
    'png',
    'wbmp',
    'webp',
    'wbmp',
    'xbm',
    'xpm',
    'gd',
];
if (PHP_VERSION_ID >= 70200) {
    $formats = array_merge($formats, [
        'bmp',
    ]);
}
$tempFile = null;
$image2 = null;

try {
    foreach ($formats as $format) {
        $loadFuntion = "imagecreatefrom{$format}";
        if (!function_exists($loadFuntion)) {
            throw new Exception("{$loadFuntion}() function is missing");
        }
        if ($format === 'xpm') {
            continue;
        }
        $saveFuntion = "image{$format}";
        if (!function_exists($saveFuntion)) {
            throw new Exception("{$saveFuntion}() function is missing");
        }
        $tempFile = tempnam(sys_get_temp_dir(), 'dpei');
        ob_start();
        if ($saveFuntion($image, $tempFile) === false) {
            throw new Exception("{$saveFuntion}() failed");
        }
        $contents = ob_get_contents();
        ob_end_clean();
        if (!is_file($tempFile)) {
            throw new Exception("{$saveFuntion}() didn't create a file");
        }
        if (filesize($tempFile) < 1) {
            if ($format !== 'xbm' || PHP_VERSION_ID >= 50600 || $contents === '') {
                throw new Exception("{$saveFuntion}() created an empty file");
            }
            file_put_contents($tempFile, $contents);
        }
        $image2 = $loadFuntion($tempFile);
        unlink($tempFile);
        $tempFile = null;
        if (!(is_resource($image2) || is_object($image2)) || imagesx($image2) !== $imageWidth || imagesy($image2) !== $imageHeight) {
            throw new Exception("{$loadFuntion}() failed");
        }
        imagedestroy($image2);
    }
} finally {
    imagedestroy($image);
    if (is_resource($image2)) {
        imagedestroy($image2);
    }
    if ($tempFile !== null) {
        unlink($tempFile);
    }
}

if (!function_exists('imagefttext')) {
    throw new Exception('imagefttext() function is missing');
}
if (!function_exists('imageantialias')) {
    throw new Exception('imageantialias() function is missing');
}

return true;