Type declarations allow functions to require that parameters are of a certain type at call time. If the given value is of the incorrect type, then an error is generated. In PHP 5, this will be a recoverable fatal error, while PHP 7 will throw a TypeError exception. We don't use or encourage using PHP < 7 in our projects.

To specify a type declaration, the type name should be added before the parameter name. The declaration can be made to accept NULL values if the default value of the parameter is set to NULL.

To enable strict typing in PHP, you need to set a declare directive at the top of your file, before the namespace definition

<?php
declare(strict_types=1);

You can type hint function arguments and return values, for example:

<?php
/**
   * Get user data
   *
   * A method that will return an array with user data.
   *
   * @param string $userToken Auth token got from url param.
   *
   * @return array             User data array.
   */
  public function getUserData(string $userToken): array
  {
      //...
  }

When the method can return more than one type (for instance, string or boolean), don't specify the return type, as this will most likely throw TypeError exceptions (you can read the RFC for mixed typehint for version 7.3 here).

Since PHP 7.1, you can explicitly declare a variable to be null

<?php
public function getArray(?string $someString): array
{
    //...
}

Type hinting is also important when working with dependency injections.

<?php
/**
 * Class User credentials
 *
 * Class that stores user credentials.
 */
class UserCredentials {
  /**
   * General Helper object
   *
   * @var string
   *
   * @since 1.0.0
   */
  private $generalHelper;

  /**
   * Initialize class
   *
   * Load helper on class init.
   *
   * @param Helper $generalHelper General helper class that implements Helper interface.
   * @since 1.0.0
   */
  public function __construct(Helper $generalHelper)
  {
      $this->generalHelper = $generalHelper;
  }

  //...
}

A table with typehints for each PHP versions: https://mlocati.github.io/articles/php-type-hinting.html