if else statement is returning always same value

I am new about PHP and ajax

Ajax return data=="OK" or data=="NO" depends on the email in the database but if-else block returns always same value.It prints always last block like "email is available!" I couldn't find the mistake.

$.ajax({
            type: 'post',
            url: 'check_email.php',
            data: {email:email,},
            success: callback});}
            function callback(data){                
                email_error = false;    
                $("#email_error").css("display","block");
                if(data =="OK"){
                    $("#email_error").html("Email was already used!");
                    $("#email_error").css("color","#990000");               
                    $("#email_error").show();
                    email_error = true; 
                }else{
                    $("#email_error").html("Email is available!");
                    $("#email_error").css("color","#F1F0D1");
                    $("#email_error").show();
                    email_error = false;
                }           
            }