Scalar types

int, bool, float, string are examples of scalar types. Scalar types represent scalar values in PHP. These types are also valid types in PHP 7.


The type scalar is the supertype of all scalar types.


array-key is the supertype (but not a union) of int and string.


positive-int allows only positive integers


numeric is a supertype of int or float and numeric-string.

class-string, interface-string

Psalm supports a special meta-type for MyClass::class constants, class-string, which can be used everywhere string can.

For example, given a function with a string parameter $class_name, you can use the annotation @param class-string $class_name to tell Psalm make sure that the function is always called with a ::class constant in that position:

class A {}

 * @param class-string $s
function takesClassName(string $s) : void {}

takesClassName("A"); would trigger a TypeCoercion issue, whereas takesClassName(A::class) is fine.

You can also parameterize class-string with an object name e.g. class-string<Foo>. This tells Psalm that any matching type must either be a class string of Foo or one of its descendants.


Psalm also supports a trait-string annotation denote a trait that exists.


Psalm also supports a enum-string annotation denote an enum that exists.


callable-string denotes a string value that has passed an is_callable check.


numeric-string denotes a string value that has passed an is_numeric check.


literal-string denotes a string value that is entirely composed of strings in your application.


  • "hello " . "world"
  • "hello " . Person::DEFAULT_NAME
  • implode(', ', ["one", "two"])
  • implode(', ', [1, 2, 3])
  • "hello " . <another literal-string>

Strings that don't pass this type check:

  • file_get_contents("foo.txt")
  • $_GET["foo"]
  • "hello " . $_GET["foo"]

lowercase-string, non-empty-string, non-empty-lowercase-string

A non empty string, lowercased or both at once.

empty here is defined as all strings except the empty string ''. Another type non-falsy-string is effectively a subtype of non-empty-string, and also precludes the string value '0'.