Is it really to use str_replace with new generated value?

I tried to replace a certain word with difference generated keys

This is my code, that replace all 1 with one similar key:

function generate($qtd){
    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $charactersLength = strlen($characters);
    $randomString = '';
    for ($i = 0; $i < $qtd; $i++) {
        $randomString .= $characters[rand(0, $charactersLength - 1)];
    }
    return $randomString;
}

$text = "nksn 1 kn ksn1dfub fui b 1uwbq k1m;lml1nfsbfjbj susksfk b1woub js sl m;sn1fjs";
$le = 10;

echo str_replace('sn', generate($le), $text);

Help please, Thanks