PHP Generics

Here’s an example of how the PHP Generics library works:

<?php

namespace App;

class Box<T> {

   private ?T $data = null;

   public function set(T $data): void {
       $this->data = $data;
   }

   public function get(): ?T {
       return $this->data;
   }
}

<?php

namespace App;

class Usage {

   public function run(): void
   {
       $stringBox = new Box<string>();
       $stringBox->set('cat');
       var_dump($stringBox->get()); // string "cat"

       $intBox = new Box<int>();
       $intBox->set(1);
       var_dump($intBox->get()); // integer 1
   }
}

The library uses monomorphization to implement generics in PHP.
You can read more about the different approaches to implementing generics.

How It Works

The library generates concrete classes from generic classes with unique names based on the name and arguments of the generic class. These classes are stored in the cache folder:

Box<string> -> BoxForString
Box<int> -> BoxForInt        

Composer then autoloads the concrete classes instead of the original generic classes.

{
   "autoload": {
       "psr-4": {
           "App\\": ["cache/","src/"]
       }
   }
}

Learn more about how the library works in this article or check out these slides.

Memory Usage

Items in collection: 10000

type	var_class_sizeof(bytes)	var_sizeof(bytes)	memory_get_usage(bytes)
array(count: 10000)	0	822,224	1,051,320
psalm(count: 10000)	1,510	822,432	1,051,560
monomorphic(count: 10000)	1,528	822,432	1,051,560
type-erased(count: 10000)	1,512	822,432	1,051,560

Performance

PHPBench (1.2.3) running benchmarks...
with configuration file: /app/phpbench.json
with PHP version 8.1.3, xdebug ❌, opcache ❌

\App\Tests\TypHintBench

    benchWithoutType........................R1 I6 - Mo240.713μs (±0.47%)
    benchWithArrayType......................R1 I70 - Mo247.663μs (±0.45%)
    benchWithMixedType......................R2 I59 - Mo249.293μs (±0.54%)
    benchWithClassType......................R1 I26 - Mo306.533μs (±0.48%)

Subjects: 4, Assertions: 0, Failures: 0, Errors: 0
+--------------+--------------------+-----+------+-----+-----------+-----------+--------+
| benchmark    | subject            | set | revs | its | mem_peak  | mode      | rstdev |
+--------------+--------------------+-----+------+-----+-----------+-----------+--------+
| TypHintBench | benchWithoutType   |     | 1000 | 100 | 674.272kb | 240.713μs | ±0.47% |
| TypHintBench | benchWithArrayType |     | 1000 | 100 | 674.272kb | 247.663μs | ±0.45% |
| TypHintBench | benchWithMixedType |     | 1000 | 100 | 674.272kb | 249.293μs | ±0.54% |
| TypHintBench | benchWithClassType |     | 1000 | 100 | 674.272kb | 306.533μs | ±0.48% |
+--------------+--------------------+-----+------+-----+-----------+-----------+--------+

PHP uses dynamic typing, so it’s no surprise that adding types slightly reduces performance. Additionally, arrays consume less memory compared to class wrappers.
You can read more about performance and memory comparisons here.