Check if email exists in sql database and output thereafter (regarding running if inside if)

So I have a form running on a page that I’m experimenting with. Atm I just have one input and that is email. Basically I want a register and if a person tries to add the same email again it will return an error msg and otherwise it will add it.

So this is the code I’ve been posting with:

<?php
    $username="user";
    $password="pass";
    $database="test";
    $email = mysql_real_escape_string( $_POST['email'] );

    mysql_connect(localhost,$username,$password);
    @mysql_select_db($database) or die( "Unable to select database");

    $query = "SELECT * FROM enroll WHERE email='$email' ";
    $result = mysql_query($query) or die(mysql_error());

    if (mysql_num_rows($result) ) {
        print 'user is already in table';
    }
    else {
        $query = "INSERT INTO enroll VALUES ('','$email')";
        $result = mysql_query($query) or die(mysql_error());       
        print 'user added';
    }
?>

That works fine! If I add an email that already exists it says “user is already in table” and if not it says “user added”.

Problem is that before I used to run another php script which validated the input and then upon submit it added it to the DB. Now it ALWAYS says “user is already in table” even before I’ve pressed Submit.

I would like to do something like this, but when posting it doesn’t work at all and I can’t understand what’s wrong.

<?php
    //If the form is submitted
    if(isset($_POST['submit'])) {

        // Required field names
        $required = array('email');

        // Loop over field names, make sure each one exists and is not empty
        $error = false;
        foreach($required as $field) {
            if (empty($_POST[$field])) {
            $error = true;
            }
        }

            if ($error) {
                echo 'You forgot to fill in your email';
            } else {
                $username="user";
                $password="pass";
                $database="test";
                $email = mysqli_real_escape_string( $_POST['email'] );

                mysqli_connect(localhost,$username,$password);
                mysqli_select_db($database) or die( "Unable to select database");

                $query = "SELECT * FROM enroll WHERE email='$email' ";
                $result = mysqli_query($query) or die(mysqli_error());

                if (mysqli_num_rows($result) ) {
                    print 'user is already in table';
                }
                else {
                    $query = "INSERT INTO enroll VALUES ('','$email')";
                    $result = mysqli_query($query) or die(mysqli_error());
                    print 'user added';
                }
            }
    }

?>

In my world, this checks if email is supplied, checks if it exists in db and if not adds it. If it exists it will come back with an error.

What am I doing wrong?

EDIT2: @ExpertSystem This is the code I’m going with now as per your recommendation:

<?php

        if(isset($_POST['submit'])) {

            // Required field names
            $required = array('email');

            // Loop over field names, make sure each one exists and is not empty
            $error = false;
            foreach($required as $field) {
                if (empty($_POST[$field])) {
                $error = true;
                }
            }

                if ($error) {
                    echo 'You forgot to fill in your email';
                } else {
                    $username="root";
                    $password="root";
                    $host="localhost";
                    $database="test";
                    $email = mysqli_real_escape_string( $_POST['email'] );

                    $link = mysqli_connect(localhost, $username, $password);
                    mysqli_select_db($link, $database) or die("Unable to select database");

                    $query = "SELECT * FROM enroll WHERE email='$email'";
                    $result = mysqli_query($link, $query) or die(mysqli_error($link));

                    if (mysqli_num_rows($result) ) {
                        print 'user is already in table';
                    }
                    else {
                        $query = "INSERT INTO enroll (email) VALUES ('$email')";
                        $result = mysqli_query($link, $query) or die(mysqli_error($link));

                        print 'user added';
                    }
                }
        }
?>