Special Types

PHPs two data types resource and NULL are classified as special types. An object of resource type refers to external resources like database connection, file streams etc. On the other hand, a NULL data type is a variable without any data assigned to it. In this chapter, we shall learn more about these types.

Resource Type

A PHP program often needs to interact with an external environment such as a database, or a disk file etc. These are treated as resources in PHP. Resource is a special data type that refers to any such external resource. PHP uses relevant functions to create these resources. For example, fopen() function opens a disk file and its reference is stored in a resource variable.

PHP’s Zend engine uses reference counting system. Hence, a resource with zero reference count is destroyed automatically by garbage collector and the memory used by resource data type need not be freed manually.

Different built-in PHP functions return respective resource variables. Subsequently, PHP uses them for interacting with the corresponding external environment. For example, the fopen() function returns a file resource, which acts as a file handle and the read/write operations on the file are facilitated by this resource variable.

The following table summarizes different functions that return resource variables −

Resource Type	Built-in functions		Definition
	Produced	Sold
bzip2	bzopen()	bzclose()	Bzip2 file
curl	curl_init()	curl_close()	Curl session
ftp	ftp_connect(),	ftp_close()	FTP stream
mssql link	mssql_connect()	mssql_close()	Link to Microsoft SQL Server database
mysql link	mysql_connect()	mysql_close()	Link to MySQL database
mysql result	mysql_db_query(),	mysql_free_result()	MySQL result
oci8 connection	oci_connect()	oci_close()	Connection to Oracle Database
ODBC link	odbc_connect()	odbc_close()	Link to ODBC database
pdf document	pdf_new()	pdf_close()	PDF document
stream	opendir()	closedir()	Dir handle
stream	fopen(), tmpfile()	fclose()	File handle
socket	socket_create()	Socket_close()	Socket handle
xml	xml_parser_create()	xml_parser_free()	XML parser
zlib	gzopen()	gzclose()	gz-compressed file
zlib.deflate	deflate_init()	None()	incremental deflate context
zlib.inflate	inflate_init()	None()	incremental inflate context

PHP has get_resource_type() function that returns resource type of a variable.

get_resource_type(resource$handle):string

where $handle is the resource variable whose type is to be obtained. This function returns a string corresponding to resource type.

There is also get_resource_id() function an integer identifier for the given resource.

get_resource_id(resource$resource):int

Example

This function provides a type-safe way for generating the integer identifier for a given resource.

<?php
   $fp = fopen("hello.php", "r");
   $resource = get_resource_type($fp);
   $id = get_resource_id($fp);
   echo "The resource type is : $resource The resource ID is : $id";
?>

It will produce the following output −

The resource type is : stream The resource ID is : 5

NULL type

In PHP, a variable with no value is said to be of null data type. Such a variable has a value defined as NULL. A variable can be explicitly assigned NULL or its value been set to null by using unset() function.

$var=NULL;

It is possible to cast variable of other type to null, although casting null to other type has been deprecated from PHP 7.2. In earlier versions, casting was done using (unset)$var syntax

Example

The following example shows how to assign NULL to a variable

<?php
   $var=NULL;
   var_dump($var);
?>

It will produce the following output −

NULL

Example

The following example performs null variable to other primary variables −

<?php
   $var = NULL;
   var_dump( (int)   $var);
   var_dump((float)$var);
   var_dump((bool)  $var) ;
   var_dump( (boolean) $var);
?>

It will produce the following output −

int(0)
float(0)
bool(false)
bool(false)