Test if the current browser runs on a mobile device.

functions.php

<?php
/**
 * Test if the current browser runs on a mobile device.
 */
function is_mobile() {
	$argent = $_SERVER['HTTP_USER_AGENT'] ? $_SERVER['HTTP_USER_AGENT'] : null;
	$mobile = false;
	if ( empty( $argent ) ) {
		$is_mobile = $mobile;
	} elseif ( strpos( $argent, 'Android' ) !== false
		|| strpos( $argent, 'BlackBerry') !== false
		|| strpos( $argent, 'Kindle') !== false
		|| strpos( $argent, 'Mobile') !== false
		|| strpos( $argent, 'Opera Mini') !== false
		|| strpos( $argent, 'Opera Mobi') !== false
		|| strpos( $argent, 'Silk/') !== false ) {
		$is_mobile = true;
	} else {
		$is_mobile = $mobile;
	}

	return $is_mobile;
}

if ( is_mobile() ) {
	echo ' Mobile ';
}else{
	echo ' Desktop ';
}