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register.php

<?php
function api_register_user( WP_REST_Request $request ) {
    $username         = sanitize_text_field( trim( $request['username'] ) );
    $email            = sanitize_email( trim( $request['email'] ) );
    $password         = sanitize_text_field( trim( $request['password'] ) );
    $confirm_password = sanitize_text_field( trim( $request['confirm_password'] ) );
    $captcha          = $request['captcha'];
    $errors = [];

    if( ! $captcha ) {
        $error = "The reCAPTCHA field is required.";
        return new WP_Error( 'captcha_error', $error, ['status' => 422] );
    }

    if( username_exists( $username ) ) {
        $errors["username"] = "Username exists already";
    }
    if( strlen( $username ) < 3 ) {
        $errors["username"] = "Username must be at least 3 characters";
    }
    if( email_exists( $email ) ) {
        $errors["email"] = "Email exists already";
    }
    if ( empty( $email ) || ! is_email( $email ) ) {
        $errors["email"] = "Valid Email is required";
    }
    if( $password !== $confirm_password ) {
        $errors["confirm_password"] = "Password confirmation don't match";
    }
    if( strlen( $password ) < 6 ) {
        $errors["password"] = "Password must be at least 6 characters";
    }
    if( count( $errors ) > 0 ) {
        return new WP_Error( 'registration_errors', $errors, ['status' => 422] );
    }

    $secret   = "dkjbdhbhjdbhjdbchjdbcjhdbchjdbchjdbchjbddhdbjbc";
    $response = json_decode(file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=".$secret."&response=".$captcha."&remoteip=".$_SERVER["REMOTE_ADDR"]), true);

    if ( $response['success'] == false ) {
        $error = "Something went wrong with reCAPTCHA validation, please try again.";
        return new WP_Error( 'captcha_error', $error, ['status' => 422] );
    }

    $user_data = [
        'user_login' =>  $username,
        'user_email' =>  $email,
        'user_pass'  =>  $password
    ];

    $user_id = wp_insert_user( $user_data );

    if( is_wp_error( $user_id ) ) {
        $error = "Sorry, registration failed due to an unxpected error. We are working on fixing it as soon as possible.";
        return new WP_Error( 'server_error', $error, ['status' => 500] );
    }
    $creds = [
        'user_login'    => $username,
        'user_password' => $password,
        'remember'      => false
    ];
    $user     = wp_signon( $creds, false );
    $username = $user->get( 'user_login' );

    $_SESSION["success"] = "Welcome $username, you have registered successfully";
    return "success";
}