over simple php backend

simple_backend.php

<?php

$_POST['action'] = 'select';
// $_POST['First'] = 'Mark';
// $_POST['Last'] = 'Twain';
// $_POST['Thumb'] = '';
// $_POST['Lat'] = '33.12343';
// $_POST['Lon'] = '-98.234563';
// $_POST['Sex'] = 'M';
// $_POST['Phone'] = '9403687777';
// $_POST['Email'] = '[email protected]';
// $_POST['Busy'] = '0'; 
// $_POST['LoggedIn'] = '1';

if(isset($_POST['action'])){
	$db = new PDO('mysql:host=localhost;dbname=mobile_web;charset=utf8', 'mobile', 'mobile');

	switch($_POST['action']){
		//Adding a new user into our most splendid spy program
		case "insert":
			$sql = "SELECT MAX(Id) as Max FROM Active_Users";
			$rows = $db->query($sql);
			$result = $rows->fetchAll(PDO::FETCH_ASSOC);
			
			$max = $result[0]['Max'] + 1;

			$sql = "INSERT INTO `mobile_web`.`Active_Users` (`Id`, `First`, `Last`, `Thumbnail`, `Lat`, `Lon`, `Sex`, `Phone`, `Email`, `Busy`, `LoggedIn`) 
					VALUES ('{$max}', '{$_POST['First']}', 
					'{$_POST['Last']}', '{$_POST['Thumb']}',
					'{$_POST['Lat']}', '{$_POST['Lon']}',
					'{$_POST['Sex']}', '{$_POST['Phone']}', 
					'{$_POST['Email']}', '{$_POST['Busy']}', '{$_POST['LoggedIn']}');";
			$db->query($sql);
		break;
		//Switching a user from either logged in or busy to vice versa.
		case "update":
		$db->query("UPDATE Active_Users SET LoggedIn = '1' WHERE ");
		break;
		
		//Remove a user completely from our spy program
		case "delete":
		break;
		
		//Getting all logged in users who are not busy
		case "select":
		
		$rows = $db->query("SELECT *  FROM Active_Users WHERE LoggedIn = '1'");
		$result = $rows->fetchAll(PDO::FETCH_ASSOC);
		print_r($result);
		break;
	}
}else{
	echo "Error:!!";
}