Definition and Usage

PHP Version
4+

The is_resource() function checks whether a given value is a resource.
It returns true if the value passed as an argument is a resource; otherwise, it returns false.

The resource type is a data type used when interacting with external resources.
Examples of resources include files, database connections, images, and network connections.

Basic Example

PHP 
/* File resource */
$file = fopen('test.txt', 'w');
var_dump(is_resource($file)); // bool(true)

/* Database connection resource */
$connection = mysqli_connect("localhost", "username", "password", "database");
var_dump(is_resource($connection)); // bool(true)

/* Image resource (when using the GD library for image creation) */
$image = imagecreate(100, 100);
var_dump(is_resource($image)); // bool(true)

/* Network connection resource */
$network = fsockopen("example.com", 80);
var_dump(is_resource($network)); // bool(true)

Note:
The gettype() function returns the data type of the given value as a string.

Syntax

PHP 
is_resource(mixed $value): bool

Parameters

Description of the is_resource() function parameters
$value Required. The value to check.

Return Values

If the value passed as an argument is a resource, the function returns true; otherwise, it returns false.

Things to Keep in Mind

There are several important points to be aware of when using the is_resource() function.

false is Returned for Closed Resource Variables

The is_resource() function is not a strict type-checking function. It returns false for closed resource variables.

The closed resource indicates that a resource previously used for interacting with an external resource has now been closed. This means the closed resource is no longer active and cannot be used.

A closed resource can be understood through the following code example:

PHP 
/* Opening a file */
$fp = fopen('test.txt', 'w');
var_dump(is_resource($fp)); // bool(true)

/* Closing the file handle */
fclose($fp); // The resource is now closed
var_dump(is_resource($fp)); // bool(false)

As shown in this code example, is_resource() returns false for closed resource variables.

To identify a closed resource, use the gettype() function. This function returns the string 'resource (closed)' for closed resources.

PHP gettype() function 'resource (closed)'
PHP 
/* Opening a file */
$fp = fopen('test.txt', 'w');

/* Closing the file handle */
fclose($fp);

echo gettype($fp); // 'resource (closed)'

Preventing Memory Leaks

When using the is_resource() function to verify the validity of a resource, the resource must be properly released after use. Failure to do so can result in performance overhead. When a resource is no longer needed, you must prevent memory leaks by using fclose() (for file handles), mysqli_close() (for MySQL connections), or other appropriate functions to release the resource.

Practical Examples

The is_resource() function is convenient and useful in the following scenarios.

Verifying the Validity of a Resource Variable

When handling external resources in PHP, such as file handles, database connections, image resources, or network connections, you can verify whether the variable is a valid resource.

PHP 
/* Opening a file */
$file = fopen('test.txt', 'r');

if (is_resource($file)) {
	// File processing code
} else {
	echo 'The file is invalid.';
}

/* Closing the file handle */
fclose($file);

Verifying the Resource Type Using xml_parser_create()

When dealing with various types of resources, you can use the is_resource() function to validate them.

PHP 
/* Creating an XML parser */
$xml_parser = xml_parser_create();

if (is_resource($xml_parser)) {
    // Parsing can be performed only if the XML parser is a valid resource
    xml_parse($xml_parser, "<tag>content</tag>", true);
    // Process results
    xml_parser_free($xml_parser);
} else {
    echo 'The XML parser resource is invalid.';
}
  • The above examples demonstrate the basic usage of the is_resource() function.
  • Since resource creation may fail depending on system conditions, it is a best practice to validate the resource before performing operations.
  • It is important to close the resource after completing the operations using it.
  • Failure to close the resource may result in memory leaks.

References

See also