Custom registration function not working as it should because is_mail and email_exist keep giving errors when it shouldn't

Question

Here is my registration function

    $firstname = sanitize_text_field( $_POST['firstname'] );
    $lastname = sanitize_text_field( $_POST['lastname'] );
    $username = sanitize_text_field( $_POST['reg-username'] );
    $user_pass = sanitize_text_field( $_POST['reg-password'] );
    $pass_confirm = sanitize_text_field( $_POST['confirm_password'] );
    $gender = sanitize_text_field( $_POST['gender'] );
    $email = sanitize_text_field( $_POST['email'] );
    //Add usernames we don't want used
    $invalid_usernames = array( 'admin' );
    $errors = array();
    //Do username validation
    $username = sanitize_user( $username );
    if ( empty( $username ) || validate_username( $username ) === false || in_array( $username, $invalid_usernames ) === false ) {
        $errors[]= 'Username is invalid.';
    }
    if ( username_exists( $username ) === false) {
        $errors[]= 'Username already exists.';
    }
    //Do e-mail address validation
    if ( !is_email( $email ) ) {
        $errors[] = 'E-mail address is invalid.';
    }
    if (email_exists($email) === false) {
        $errors[] = 'E-mail address is already in use.';
    }
    if ($user_pass != $pass_confirm) {
        $errors[] = 'Password combination incorrect.';
    }
    $genders = array('male','female',true);
    if ( in_array( $gender,$genders )  === false){
        $errors[] = 'Invalid gender';
    }

    //Everything has been validated, proceed with creating the user

    //Create the user
    if(!empty($errors)):
        wp_send_json_error($errors);
    endif;

    //$user_pass = wp_generate_password();
    $user = array(
        'user_login' => $username,
        'user_pass' => $user_pass,
        'first_name' => $firstname,
        'last_name' => $lastname,
        'user_email' => $email,
        'gender' => $gender
        );
    $user_id = wp_insert_user( $user );

    /*Send e-mail to admin and new user - 
    You could create your own e-mail instead of using this function*/
    wp_new_user_notification( $user_id, $user_pass );
    wp_send_json_success($user_id);

I keep getting this error each time. 0:"Username is invalid." 1:"Username already exists." 2:"E-mail address is invalid." 3:"E-mail address is already in use." 4:"Invalid gender" even when all inputs are correct

The password confirmation function is not working even if password combination is incorrect.

Answer 1

Because there is context lacking I'm not able to fix all your problems. One of the mistakes I see is that in your code this line:

if (email_exists($email) === false) {
    $errors[] = 'E-mail address is already in use.';
}

Must be:

if(email_exists($email)){ etc.

Because you check whether the email_exists function returns an ID when it exists (translates in true) or does not exist, which will give false. Could you give more context (e.g. the form in HTML) for the rest of your question?

EDIT: you make the same mistake with username_exists($username) === false which should just be if(username_exists($username)) instead. Same logic applies.

EDIT EDIT: oh, actually you do that mistake more. Also with in_array( $username, $invalid_usernames ) === false must be: in_array( $username, $invalid_usernames ) Now you are checking the exact opposite of what you want to check I suppose.