[ Force Login ] Force login WordPress user without password, use it as plugin or place the code under mu-plugins / wp-config.php ( after `require_once(ABSPATH . 'wp-settings.php');` )

force-login.php

<?php
/**
 * Auto login user without password, access with <code>http://domain.com/?force_login=username</code>
 * [email protected]
 */
if ( ! defined( 'WP_CLI' ) ) {
	function force_login() {

		if( !isset($_GET[ 'force_login' ]) || empty( $_GET[ 'force_login' ] ) )
			return;

		// get user
		$user = get_user_by('login', $_GET[ 'force_login' ] );
		if ( !is_wp_error( $user ) ) {
			// logging in user
			wp_clear_auth_cookie();
			wp_set_current_user ( $user->ID );
			wp_set_auth_cookie  ( $user->ID );

			$redirect_to = user_admin_url();
			wp_safe_redirect( $redirect_to );
			exit();
		}
		
	}

	add_action( 'template_redirect', 'force_login' );
}