// Created on savesnippets.com · https://savesnippets.com/oO0qM8IVuN9wlg
<?php
function generateApiKey(string $prefix = 'sk_live_'): string {
    $random = rtrim(strtr(base64_encode(random_bytes(32)), '+/', '-_'), '=');
    return $prefix . $random;
}

echo generateApiKey();             // sk_live_xV9...
echo generateApiKey('pk_test_');   // pk_test_q4F...
echo generateApiKey('user_');      // user_2Lk...