<?php

function urlsafe_base64_encode($string)
{
    return str_replace(['+','/','='], ['-','_',''], base64_encode($string));
}
function urlsafe_base64_decode($string)
{
    $string = str_replace(['-','_'], ['+','/'], $string);
    $mod4 = strlen($string) % 4;
    if ($mod4) $string .= substr('====', $mod4);
    return base64_decode($string);
}

/** test code */

$text = "Smile for me 😃..";
$encode = urlsafe_base64_encode($text);
$decode = urlsafe_base64_decode($encode);

echo "
String: $text
Encoded: $encode
Decoded: $decode
";

if($text == $decode) echo "Validated." . PHP_EOL;