Description:
------------
SELECT EXISTS in the query fills up the memory. see test-script below.

Test script:
---------------
<?php
$start = microtime(true);
var_dump(memory_get_usage(true));

for ($i = 0;$i<100000;$i++) {
    $sql = 'SELECT 1
            FROM `article`
            WHERE a_nr = "'.$mysqli->real_escape_string(bin2hex(random_bytes(10))).'"
            LIMIT 1';
    $mysqli->query($sql);
}

var_dump(microtime(true) - $start, memory_get_usage(true));
$start = microtime(true);

for ($i = 0;$i<100000;$i++) {
    $sql = 'SELECT EXISTS
            (
                SELECT 1
                FROM `article`
                WHERE a_nr = "'.$mysqli->real_escape_string(bin2hex(random_bytes(10))).'"
                LIMIT 1
            )';
    $mysqli->query($sql);
}

var_dump(microtime(true) - $start, memory_get_usage(true));

Expected result:
----------------
memory1: int(4194304)
memory2: int(4194304)
momory3: int(28311552)

Actual result:
--------------
php memory fills up in the second query