Can We Have Static Types in PHP without PHP 7 or HHVM?

This article explores the challenges of type safety in PHP 5 and introduces the "augmented types" extension as a solution. While PHP 7 offers built-in scalar type declarations, this extension provides similar functionality for older PHP versions.

The Problem: Weak Typing in PHP 5

PHP 5's dynamic typing can lead to unexpected behavior. Consider a function designed to increment a number:

function plusone($a) { return $a + 1; }

This function accepts various input types and performs implicit type conversion. Adding robust type checking requires manual validation:

function plusone($a) { if (!is_numeric($a)) throw new InvalidArgumentException("Only numbers!"); return $a + 1; }

PHP 5 also lacks automatic return type enforcement.

Augmented Types: A PHP 5 Solution

The Box team's "augmented types" extension leverages phpDoc annotations to enforce type safety at runtime. It avoids core modifications or forking.

Installation (for PHP 5):

Follow these steps (Ubuntu example, adapt for other systems):

1. Update system: sudo apt-get update
2. Install dependencies: sudo apt-get install php5-dev bison flex
3. Clone the repo: git clone git@github.com:box/augmented_types.git
4. Install the extension: phpize; ./configure --enable-augmented_types; make; make test; sudo make install
5. Enable the extension in php.ini (find its location using php -i | grep 'Loaded Configuration File'): Add zend_extension=/path/to/augmented_types.so.

Enable the extension per-file with ini_set("augmented_types.enforce_by_default", 1); or globally in php.ini.

Usage:

Augmented types uses phpDoc for type declarations. For example:

/**
 * @param int $a
 * @return int
 */
function plusone($a) { return $a + 1; }

Now, passing a non-integer to plusone() will result in a runtime error. The extension supports composite types (@param int|float $a), array types (@param int[] $a), and enforcing return types, including @return void.

Ignoring Files:

To prevent errors from external libraries, use augmented_types_blacklist() or the augmented_types.blacklist and augmented_types.whitelist directives in php.ini to manage which files are checked.

PHP 7 and Beyond:

PHP 7's built-in type hinting renders this extension largely unnecessary for new projects. However, "augmented types" offers features not found in PHP 7, such as composite and array type declarations. Remember that it adds runtime overhead and should be used primarily during development and testing.

Conclusion:

While PHP 7 provides superior type safety, the "augmented types" extension offers a valuable solution for maintaining type safety in legacy PHP 5 applications. Its extra features come at the cost of performance, making it ideal for development and testing but not for production environments.

Frequently Asked Questions (FAQs): (These are already well-covered in the original text, so I won't repeat them here. The original FAQ section is comprehensive.)

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