Question

Other languages with automatic variable declaration – like Perl – have a strict mode.

By activating this strict mode, variable declaration is required, and Perl throws an error as soon as you try to use an undeclared variable.

Does PHP offer a similar feature?

Answer 1

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

 ?>