how to restart/clear memcache without restarting the whole web server?

yes. you can clear the memcache. try:

telnet localhost 11211
flush_all
quit

if the memcache does not run on localhost 11211, you will have to adjust it.

This will also work using netcat

echo "flush_all" | nc -q 2 localhost 11211 

Then just wait for the "OK".

memcflush in the memcache tools is what you want:

memcflush --servers=localhost:11211

Change localhost to whatever your server is.

The memcache tools may not be installed on the server, if you're running a Debian-based OS you can install it like this:

sudo apt-get install libmemcached-tools

In Bash you may use this fancy syntax:

echo flush_all > /dev/tcp/localhost/11211

Otherwise use memflush command:

memflush --servers=localhost

(sleep 2; echo flush_all; sleep 2; echo quit; ) | telnet 127.0.0.1 11211

if you want to run it non-interactively

thanks to @heiko

Rather than waiting for timeouts you can make the command instantaneous by following flush_all with the quit command:

printf "flush_all\r\nquit\r\n" | nc localhost 11211

Alternatively if you don't have nc:

printf "flush_all\r\nquit\r\n" > /dev/tcp/127.0.0.1/11211

Though this method won't produce an output, although you can verify it works by checking stats to see that cmd_flush increased.

in case you use a socket for connecting to memcached the syntax is

echo "flush_all" | nc -U ~/memcached.sock

Here is a function to flush memcached via PHP, in case that you need to refresh it without logging into ssh...

You can just http://yourserver.com/memflush.php

Call this file memflush.php

<?php

 $socket = fsockopen("localhost", "11211", $errno, $errstr); 

    if($socket) { 
        echo "Connected. <br /><br />"; 
    }
    else { 
        echo "Connection failed!<br /><br />"; 
    }

    fputs($socket, "flush_all\r\n"); 
    $buffer = ""; 

    fclose($socket); 
?>