// initiate the memcached instance
$cache = new \Memcached();
$cache->addServer('localhost', '11211');

// get all stored memcached items

$keys = $cache->getAllKeys();
$cache->getDelayed($keys);

$store = $cache->fetchAll();

// delete by regex keys

$keys = $cache->getAllKeys();
$regex = 'product_.*';
foreach($keys as $item) {
    if(preg_match('/'.$regex.'/', $item)) {
        $cache->delete($item);
    }
}

The right way to dump slab keys seems to be by using lru_crawler metadump instead of stats cachedump, see https://github.com/memcached/memcached/issues/405

<?php

function getAllKeys(string $host, int $port): array
{
    $sock = fsockopen($host, $port, $errno, $errstr);
    if ($sock === false) {
        throw new Exception("Error connection to server {$host} on port {$port}: ({$errno}) {$errstr}");
    }

    if (fwrite($sock, "stats items\n") === false) {
        throw new Exception("Error writing to socket");
    }

    $slabCounts = [];
    while (($line = fgets($sock)) !== false) {
        $line = trim($line);
        if ($line === 'END') {
            break;
        }

        // STAT items:8:number 3
        if (preg_match('!^STAT items:(\d+):number (\d+)$!', $line, $matches)) {
            $slabCounts[$matches[1]] = (int)$matches[2];
        }
    }

    foreach ($slabCounts as $slabNr => $slabCount) {
        if (fwrite($sock, "lru_crawler metadump {$slabNr}\n") === false) {
            throw new Exception('Error writing to socket');
        }

        $count = 0;
        while (($line = fgets($sock)) !== false) {
            $line = trim($line);
            if ($line === 'END') {
                break;
            }

            // key=foobar exp=1596440293 la=1596439293 cas=8492 fetch=no cls=24 size=14908
            if (preg_match('!^key=(\S+)!', $line, $matches)) {
                $allKeys[] = $matches[1];
                $count++;
            }
        }

//        if ($count !== $slabCount) {
//            throw new Exception("Surprise, got {$count} keys instead of {$slabCount} keys");
//        }
    }

    if (fclose($sock) === false) {
        throw new Exception('Error closing socket');
    }
    
    return $allKeys;
}

First I use the lastest memcached version 1.4.25, but unfortunately I found memcached::getAllkeys do not work with it, though i follow the others suggestion to disable Memcached::OPT_BINARY_PROTOCOL. So i try to use history versions, when i use memcached version 1.4.17, it works.

/**
 * Get all memcached keys. Special function because getAllKeys() is broken since memcached 1.4.23. Should only be needed on php 5.6
 *
 * cleaned up version of code found on Stackoverflow.com by Maduka Jayalath
 *
 * @return array|int - all retrieved keys (or negative number on error)
 */
public function getMemcachedKeys($host = '127.0.0.1', $port = 11211)
{
    $mem = @fsockopen($host, $port);
    if ($mem === false)
    {
        return -1;
    }

    // retrieve distinct slab
    $r = @fwrite($mem, 'stats items' . chr(10));
    if ($r === false)
    {
        return -2;
    }

    $slab = [];
    while (($l = @fgets($mem, 1024)) !== false)
    {
        // finished?
        $l = trim($l);
        if ($l == 'END') 
        {
            break;
        }

        $m = [];
        // <STAT items:22:evicted_nonzero 0>
        $r = preg_match('/^STAT\sitems\:(\d+)\:/', $l, $m);
        if ($r != 1) 
        {
            return -3;
        }
        $a_slab = $m[1];

        if (!array_key_exists($a_slab, $slab)) 
        {
            $slab[$a_slab] = [];
        }
    }

    reset($slab);
    foreach ($slab as $a_slab_key => &$a_slab)
    {
        $r = @fwrite($mem, 'stats cachedump ' . $a_slab_key . ' 100' . chr(10));
        if ($r === false) 
        {
            return -4;
        }

        while (($l = @fgets($mem, 1024)) !== false)
        {
            // finished?
            $l = trim($l);
            if ($l == 'END') 
            {
                break;
            }

            $m = [];
            // ITEM 42 [118 b; 1354717302 s]
            $r = preg_match('/^ITEM\s([^\s]+)\s/', $l, $m);
            if ($r != 1) 
            {
                return -5;
            }
            $a_key = $m[1];

            $a_slab[] = $a_key;
        }
    }

    // close the connection
    @fclose($mem);
    unset($mem);

    $keys = [];
    reset($slab);
    foreach ($slab AS &$a_slab)
    {
        reset($a_slab);
        foreach ($a_slab AS &$a_key) 
        {
            $keys[] = $a_key;
        }
    }
    unset($slab);

    return $keys;
}