Google App Engine permits only specific whitelisted extensions to be used within PHP applications. If you use a non-whitelisted extension, you’ll see the below error:
The [php_extension_name] extension is missing. Please check your PHP configuration.
If you need a certain extension for your PHP application, ensure that it’s enabled in GAE: check the official list at https://developers.google.com/appengine/docs/php/#PHP_Enabled_extensions . If your preferred extension is not listed, you can also try searching for a pure-PHP implementation (Pure PHP extensions can always be uploaded as part of an application; C based extensions must be whitelisted.)
If you need an extension not listed in the above link, you can request it via the App Engine issues tracker. For example, here’s a feature request for the ImageMagick extension: https://code.google.com/p/googleappengine/issues/detail?id=9424