Strict mode in PHP

Kind of. You can activate the E_NOTICE level in your error reporting. (List of constants here.)

Every instance of usage of an undeclared variable will throw an E_NOTICE.

The E_STRICT error level will also throw those notices, as well as other hints on how to optimize your code.

error_reporting(E_STRICT);

Terminating the script

If you are really serious, and want your script to terminate instead of just outputting a notice when encountering an undeclared variable, you could build a custom error handler.

A working example that handles only E_NOTICEs with "Undefined variable" in them and passes everything else on to the default PHP error handler:

<?php

error_reporting(E_STRICT);

function terminate_missing_variables($errno, $errstr, $errfile, $errline)
{                               
  if (($errno == E_NOTICE) and (strstr($errstr, "Undefined variable")))
   die ("$errstr in $errfile line $errline");

  return false; // Let the PHP error handler handle all the rest  
}

$old_error_handler = set_error_handler("terminate_missing_variables"); 

echo $test; // Will throw custom error

xxxx();  // Will throw standard PHP error

 ?>

Use

error_reporting(-1);

to show every possible error, including E_STRICT and even when new levels and constants are added in future PHP versions.

(Reference)

After some years, PHP 7.0.0 has gained declare(strict_types=1).

Yes, type error_reporting(E_STRICT|E_ALL); in the beginning of your script.