check back in $.post

check back in $.post

Guys, good morning!

I'm in trouble and need your help. My situation is as follows:
In the onclick of a form button, I call a function called register (below code)

function register(){
    $.post("register.php",{usu_email : $("#register_usu_email").val()}, function(data) {
        if(data === 'yes'){
            ...
        }else{
            ...
        }
    });
};

The goal is to verify if the email is already registered in the database, if so, returns a 'yes', and do a script for error.

Within my register.php I check as follows below. My problem is that the register function, the if (data === 'liked') is never true, as much as my user has already registered in the database, if I take a alert (date) it shows to me 'yes', but if I put it in a condition, it is never true. How do I treat a correct return in this situation?

<?php
    include 'conexao/conexao.php';
 
    $usu_email    = $_POST['usu_email'];
 
    $sql = "select * from usuario where usuario.usu_email = '$usu_email'";
    
    $retorno = mysql_query($sql,$mysql);
 
    echo "email";
    
    if (mysql_num_rows($retorno) == 1){
        echo "achou";
    } else{
        echo "nao achou";
    }