str_random.php

<?php

function str_random($length = 16)
{
    $string = '';
    $len = strlen($string);

    while ($len < $length) {
        $size = $length - $len;

        $bytes = random_bytes($size);
        $bytes = base64_encode($bytes);
        
        $blackList = ['/', '+', '='];

        $cleanStr = str_replace($blackList, '', $bytes);
        
        $string .= substr($cleanStr, 0, $size);
        
        $len = strlen($string);
    }

    return $string;
}