How to check if memcache or memcached is installed for PHP?

Note that all of the class_exists, extensions_loaded, and function_exists only check the link between PHP and the memcache package.

To actually check whether memcache is installed you must either:

• know the OS platform and use shell commands to check whether memcache package is installed
• or test whether memcache connection can be established on the expected port

EDIT 2: OK, actually here's an easier complete solution:

if (class_exists('Memcache')) {
    $memcache = new Memcache;
    $isMemcacheAvailable = @$memcache->connect('localhost');
}
if ($isMemcacheAvailable) {
    //...
}

Outdated code below

EDIT: Actually you must force PHP to throw error on warnings first. Have a look at this SO question answer.

You can then test the connection via:

try {
    $memcache->connect('localhost');
} catch (Exception $e) {
    // well it's not here
}