Não há necessidade de usar pecl. Instale o módulo de extensão memcached para PHP5:
$ sudo apt-get install php5-memcached
test.php:
<?php
$m = new Memcached();
$m->addServer('127.0.0.1', 11211);
var_dump($m->getStats());
?>
$ php test.php
array(1) {
["127.0.0.1:11211"]=>
array(24) {
["pid"]=>
int(2462)
["uptime"]=>
int(129)
Para testar o memcached, use o telnet:
$ telnet 127.0.0.1 11211
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
stats
STAT pid 2462
STAT uptime 1039
STAT time 1298284545
STAT version 1.4.5
STAT pointer_size 32
STAT rusage_user 0.092005
STAT rusage_system 0.000000