Types

General

The meaning of a value is determined by its type. PHP’s types are categorized as scalar types and composite types. The scalar types are Boolean, integer, floating point, string, and null. The composite types are array, and object. The resource is an opaque type whose internal structure is not specified and depends on the implementation.

The scalar types are value types. That is, a variable of scalar type behaves as though it contains its own value.

The composite types can contain other variables, besides the variable itself, e.g. array contains its elements and object contains its properties.

The objects and resources are handle types. The type contains information — in a handle — that leads to the value. The differences between value and handle types become apparent when it comes to understanding the semantics of assignment, and passing arguments to, and returning values from, functions.

Variables are not declared to have a particular type. Instead, a variable’s type is determined at runtime by the value it contains. The same variable can contain values of different types at different times.

Useful library functions for interrogating and using type information include gettype, is_type, settype, and var_dump.

Scalar Types

General

The integer and floating-point types are collectively known as arithmetic types. The library function is_numeric indicates if a given value is a number or a numeric string.

The library function is_scalar indicates if a given value has a scalar type. However, that function does not consider NULL to be scalar. To test for NULL, use is_null.

Some objects may support arithmetic and other scalar operations and/or be convertible to scalar types (this is currently available only to internal classes). Such object types together with scalar types are called scalar-compatible types. Note that the same object type may be scalar-compatible for one operation but not for another.

The Boolean Type

The Boolean type is bool, for which the name boolean is a synonym. This type is capable of storing two distinct values, which correspond to the Boolean values true and false, respectively. The internal representation of this type and its values is unspecified.

The library function is_bool indicates if a given value has type bool.

The Integer Type

There is one integer type, int, for which the name integer is a synonym. This type is binary, signed, and uses twos-complement representation for negative values. The range of values that can be stored is implementation-defined; however, the range [-2147483648, 2147483647], must be supported. This range must be finite.

Certain operations on integer values produce a mathematical result that cannot be represented as an integer. Examples include the following:

• Incrementing the largest value or decrementing the smallest value.
• Applying the unary minus to the smallest value.
• Multiplying, adding, or subtracting two values.

In such cases, the computation is done as though the types of the values were float with the result having that type.

The constants PHP_INT_SIZE, PHP_INT_MIN and PHP_INT_MAX define certain characteristics about type int.

The library function is_int indicates if a given value has type int.

The Floating-Point Type

There is one floating-point type, float, for which the names double and real are synonyms. The float type must support at least the range and precision of IEEE 754 64-bit double-precision representation.

The library function is_float indicates if a given value has type float. The library function is_finite indicates if a given floating-point value is finite. The library function is_infinite indicates if a given floating-point value is infinite. The library function is_nan indicates if a given floating-point value is a NaN.

The String Type

A string is a set of contiguous bytes that represents a sequence of zero or more characters.

Conceptually, a string can be considered as an array of bytes—the elements—whose keys are the int values starting at zero. The type of each element is string. However, a string is not considered a collection, so it cannot be iterated over.

A string whose length is zero is an empty string.

As to how the bytes in a string translate into characters is unspecified.

Although a user of a string might choose to ascribe special semantics to bytes having the value \0, from PHP’s perspective, such null bytes have no special meaning. PHP does not assume strings contain any specific data or assign special values to any bytes or sequences. However, many library functions assume the strings they receive as arguments are UTF-8 encoded, often without explicitly mentioning that fact.

A numeric string is a string whose content exactly matches the pattern defined by the str-numeric production below. A leading-numeric string is a string whose initial characters follow the requirements of a numeric string, and whose trailing characters are non-numeric. A non-numeric string is a string that is not a numeric string.

str-numeric::
   str-whitespaceopt   signopt   str-number

str-whitespace::
   str-whitespaceopt   str-whitespace-char

str-whitespace-char::
   new-line
   Space character (0x20)
   Horizontal-tab character (0x09)
   Vertical-tab character (0x0B)
   Form-feed character (0x0C)

str-number::
   digit-sequence
   floating-literal

Note that digit-sequence is interpreted as having base-10 (so "0377" is treated as 377 decimal with a redundant leading zero, rather than as octal 377).

Only one mutation operation may be performed on a string, offset assignment, which involves the simple assignment operator =.

The library function is_string indicates if a given value has type string.

The Null Type

The null type has only one possible value, NULL. The representation of this type and its value is unspecified.

The library function is_null indicates if a given value is NULL.

Composite Types

The Array Type

An array is a data structure that contains a collection of zero or more elements whose values are accessed through keys that are of type int or string. See more details in arrays chapter.

The library function is_array indicates if a given value is an array.

Objects

An object is an instance of a class. Each distinct class-declaration defines a new class type, and each class type is an object type. The representation of object types is unspecified.

The library function is_object indicates if a given value is an object, and the library function get_class indicates the name of an object’s class.

Resources

A resource is a descriptor to some sort of external entity. Examples include files, databases, and network sockets.

A resource is an abstract entity whose representation is unspecified. Resources are only created or consumed by the implementation; they are never created or consumed by PHP code.

Each distinct resource has a unique identity of some unspecified form.

The library function is_resource indicates if a given value is a resource, and the library function get_resource_type indicates the type of a resource.