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

You can look at phpinfo() or check if any of the functions of memcache is available. Ultimately, check whether the Memcache class exists or not.

e.g.

if(class_exists('Memcache')){  // Memcache is enabled.}

why not use the extension_loaded() function?

Use this code to not only check if the memcache extension is enabled, but also whether the daemon is running and able to store and retrieve data successfully:

<?phpif (class_exists('Memcache')) {    $server = 'localhost';    if (!empty($_REQUEST['server'])) {        $server = $_REQUEST['server'];    }    $memcache = new Memcache;    $isMemcacheAvailable = @$memcache->connect($server);    if ($isMemcacheAvailable) {        $aData = $memcache->get('data');        echo '<pre>';        if ($aData) {            echo '<h2>Data from Cache:</h2>';            print_r($aData);        } else {            $aData = array(                'me' => 'you',                'us' => 'them',            );            echo '<h2>Fresh Data:</h2>';            print_r($aData);            $memcache->set('data', $aData, 0, 300);        }        $aData = $memcache->get('data');        if ($aData) {            echo '<h3>Memcache seem to be working fine!</h3>';        } else {            echo '<h3>Memcache DOES NOT seem to be working!</h3>';        }        echo '</pre>';    }}if (!$isMemcacheAvailable) {    echo 'Memcache not available';}?>