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<?php
//Register login route
//Test in postman with - www.yourdomain.com/wp-json/login-user/v1/user
function uab_register_endpoints()
{
    register_rest_route('login-user/v1', '/user/', array(
        'methods' => 'POST',
        'callback' => 'uab_login_user' 
    ));

}
add_action('rest_api_init', 'uab_register_endpoints');

//Callback function for handling login. 
function uab_login_user(WP_REST_Request $request)
{
    $username = sanitize_text_field( trim( $request['username'] ) );
    $password = trim( $request['password'] );
    $remember = $request['remember'];
    $creds = array(
        'user_login'    => $username,
        'user_password' => $password,
        'remember'      => $remember
    );
    //If wp_signon fails it will return an error.
    $user = wp_signon( $creds, false );
    if ( is_wp_error( $user ) )
    {
        $error = "Invalid username and password combination";
        return new WP_Error( 'login_error', $error, array( 'status' => 422 ) );
    }
    return "Welcome back $username!";
}