Since PHP 7, type declarations have become a cornerstone of robust PHP code. Type hints let you declare the expected types of function parameters and return values, enabling PHP's runtime to catch mismatches before they cause subtle bugs. For instance, declaring a function that accepts two int parameters and returns an int ensures that the caller respects the contract. When types do not match, PHP throws a TypeError rather than silently coercing the data.
By default PHP operates in coercive mode, attempting to convert values into the expected type whenever possible. You can opt into stricter behavior by placing declare(strict_types=1); at the top of a file, which forces PHP to throw a TypeError on any type mismatch without attempting coercion. Strict mode is widely recommended for new projects because it eliminates ambiguity and surfaces bugs earlier in the development cycle.
PHP 8 expanded the type system further. Union types let you declare that a parameter or return value may be one of several listed types, written as TypeA|TypeB. This is useful when a function legitimately accepts multiple input forms, such as a string or integer that both can be processed. Closely related is the nullable type syntax, where prefixing a type with a question mark, as in ?User, indicates that the value may be the specified type or null. Together, these features allow you to express precise contracts at the boundary between components, catching misuse while still accommodating realistic data shapes.