What type system is used in PHP?

PHP uses a dynamic and weak type system.

• Dynamic typing: The type of a variable is determined at runtime during assignment, not at declaration. A variable can hold values of different types throughout its lifecycle.

  <?php
  $variable = 10;      // integer
  echo gettype($variable) . "\n";
  
  $variable = "hello"; // string
  echo gettype($variable) . "\n";
  ?>
  

• Weak (non-strict) typing: PHP automatically converts data types when necessary, such as during arithmetic operations or comparisons. This can lead to unexpected results if types are not explicitly controlled.

  <?php
  echo "10" + 5; // Output: 15 (string "10" converted to number)
  echo "\n";
  if ("5" == 5) { // Comparison of "5" and 5 with automatic type conversion
      echo "Equal"; // Output: Equal
  }
  ?>
  

However, starting from PHP 7, it became possible to use:

• Scalar type declarations: Allow specifying the expected type of function arguments, return values, and class properties (PHP 7.4+).

  <?php
  declare(strict_types=1); // Strict mode for the current file
  
  function addNumbers(int $a, int $b): int {
      return $a + $b;
  }
  
  // addNumbers("1", 2); // This will cause a TypeError in strict mode
  echo addNumbers(1, 2);
  ?>
  

• Return type declarations: Specify the data type that a function should return.

• Property type declarations: Specify the data type that a class property can hold.

• Union types: Allow indicating that a variable, argument, property, or return value can be one of several types (PHP 8+).

• Intersection types: Indicate that a variable, argument, property, or return value must be an instance of all specified interfaces or classes (PHP 8.1+).

• mixed type: Indicates that an element can be of any type (PHP 8+).

• never type: Indicates that a function or method never returns a value (i.e., either terminates execution or throws an exception) (PHP 8.1+).

Using these features allows PHP to be partially closer to strict typing, improving code readability and debugging. Applying declare(strict_types=1); at the beginning of a file enables strict mode for scalar types, where automatic type coercion during function and method calls does not occur.