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What is the purpose of the declare(strict_types=1); directive in PHP?
What is the purpose of the declare(strict_types=1); directive in PHP?
Jun 11, 2025 am 12:15 AM
Strict type checking only affects the type declaration of function parameters, and does not affect the return type and internal functions. After strict_types=1 is enabled, PHP will force the function parameter type to match, otherwise TypeError will be raised; but it has no effect on the return type and built-in function. It must be declared separately at the top of each file, not inherited or effective globally. Common misunderstandings include: mistakenly thinking that it affects the return type, takes effect globally, or ignores numeric strings and can still be converted to numeric types. User input should be manually verified or converted during development to avoid type errors.
The declare(strict_types=1); directive in PHP enables strict type checking for function arguments in the file it's declared in. Without it, PHP will try to coerce values ??into the correct type if possible. With it, PHP will throw a TypeError if the value passed is not of the expected type.
What Strict Types Actually Affect
Strict types only affect type declarations in function calls — that include method parameters, closings, and function arguments. It does not affect scalar return types or internal PHP functions.
For example:
declare(strict_types=1);
function add(int $a, int $b): int {
return $a $b;
}
add(1, 2); // Works fine
add("1", "2"); // Also works because strings are coerced to integers
add('a', 'b'); // Throws TypeError In this case, passing 'a' and 'b' throws an error because they can't be converted to integers. But numeric strings like "1" still work because PHP knows how to convert them.
Where You Need to Use It
You need to include declare(strict_types=1); at the top of every PHP file where you want strict typing behavior. It's not inherited across files or namespaces.
- It must be the first statement in the file — no code or whitespace before it.
- It applies only to the current file, even if that file is included or required by another.
So if you're building a library or application with consistent typing, you should make sure each file starts with this directive.
Common Mistakes and Gotchas
Here are some common misunderstandings or pitfalls when using strict types:
- ? Mixing types without proper casting leads to errors.
- ? Assuming
strict_typesaffects return types — it doesn't (but return types are still checked). - ? Thinking it's enabled globally — it's always file-level only.
- ? Forgetting that numeric strings like
"123"are still considered valid whenintis expected.
Also, keep in mind that during development, especially when working with APIs or user input, you might need to explicitly cast or validate inputs before passing them to typed functions.
It's a small line of code but makes a big difference in how PHP handles type safety. Once you start using it consistently, you'll likely notice fewer bugs related to unexpected data types.
That's basically all there is to it — straightforward, but easy to overlook key details like placement and scope.
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