Types: Type Conversion

In a few situations (documented in the following sections) the values of operands are implicitly converted from one type to another. Explicit conversion is performed using the cast operator.

If an expression is converted to its own type, the type and value of the result are the same as the type and value of the expression.

When an expression of type num is converted, if that expression is currently an int, then int conversion rules apply; otherwise, float conversion rules apply.

When an expression of type arraykey is converted, if that expression is currently an int, then int conversion rules apply; otherwise, string conversion rules apply.

Conversion to bool

No non-bool type can be converted implicitly to bool. All other conversions must be explicit.

If the source type is int or float, then if the source value tests equal to 0, the result value is false; otherwise, the result value is true.

If the source value is null, the result value is false.

If the source is an empty string or the string "0", the result value is false; otherwise, the result value is true.

If the source is an array with zero elements, the result value is false; otherwise, the result value is true.

If the source is an object, the result value is true, with some legacy exceptions:

  • SimpleXMLElement can be false if there are no child elements
  • The legacy Hack Collection classes Vector, Map, Set and their immutable variants can be false if there are no elements in the collection.

We strongly recommend not depending on these behaviors.

If the source is a resource, the result value is false.

The library function boolval allows values to be converted to bool.

Conversion to int

No non-int type can be converted implicitly to int. All other conversions must be explicit.

If the source type is bool, then if the source value is false, the result value is 0; otherwise, the result value is 1.

If the source type is float, for the values INF, -INF, and NAN, the result value is implementation-defined. For all other values, if the precision can be preserved, the fractional part is rounded towards zero and the result is well defined; otherwise, the result is undefined.

If the source value is null, the result value is 0.

If the source is a numeric string or leading-numeric string having integer format, if the precision can be preserved the result value is that string's integer value; otherwise, the result is undefined. If the source is a numeric string or leading-numeric string having floating-point format, the string's floating-point value is treated as described above for a conversion from float. The trailing non-numeric characters in leading-numeric strings are ignored. For any other string, the result value is 0.

If the source is an array with zero elements, the result value is 0; otherwise, the result value is 1.

If the source is a resource, the result is the resource's unique ID.

The library function intval allows values to be converted to int.

Converting to float

No non-float type can be converted implicitly to float. All other conversions must be explicit.

If the source type is int, if the precision can be preserved the result value is the closest approximation to the source value; otherwise, the result is undefined.

If the source is a numeric string or leading-numeric string having integer format, the string's integer value is treated as described above for a conversion from int. If the source is a numeric string or leading-numeric string having floating-point format, the result value is the closest approximation to the string's floating-point value. The trailing non-numeric characters in leading-numeric strings are ignored. For any other string, the result value is 0.

If the source is an array with zero elements, the result value is 0.0; otherwise, the result value is 1.0.

If the source is a resource, the result is the resource's unique ID.

The library function floatval allows values to be converted to float.

Converting to num

The only implicit conversions to type num are from the types int and float. There is no change in representation during such conversions. There are no explicit conversions.

Converting to string

Except for the type classname, no non-string type can be converted implicitly to string. All other conversions must be explicit.

If the source type is bool, then if the source value is false, the result value is the empty string; otherwise, the result value is "1".

If the source type is int or float, then the result value is a string containing the textual representation of the source value (as specified by the library function sprintf).

If the source value is null, the result value is an empty string.

If the source is an object, then if that object's class has a __toString method, the result value is the string returned by that method; otherwise, the conversion is invalid.

If the source is a resource, the result value is an implementation-defined string.

If the source type is the classname type, the result value is a string containing the corresponding fully qualified class or interface name without any leading \.

The library function strval allows values to be converted to string.

Converting to arraykey

The only implicit conversions to type arraykey are from the types int and string. There is no change in representation during such conversions. There are no explicit conversions.

Converting to an Object Type

An object type can be converted implicitly to any object type from which the first object type is derived directly or indirectly. There are no other implicit or explicit conversions.

Converting to an Interface Type

An object type can be converted implicitly to any interface type that object type implements directly or indirectly.

An interface type can be converted implicitly to any interface type from which the first interface type is derived directly or indirectly.

There are no other implicit or explicit conversions.

Converting to Resource Type

The predefined resource-like constants STDIN, STDOUT, and STDERR, can be converted implicitly to resource. No other non-resource type can be so converted. No explicit conversions exist.

Converting to Mixed Type

Any type can be converted implicitly to mixed. No explicit conversions exist.

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