The assertNull() function is a builtin function in PHPUnit and is used to assert whether the variable is Null or not. This assertion will return true in the case if the variable is Null else return false. In case of true the asserted test case got passed else test case got failed.
Syntax:
assertNull(mixed $variable[, string $message = ''])
Parameters: This function accepts two parameters as mentioned above and described below:
- $Variable: This parameter represents any type of variable.
- $message: This parameter takes a string value. When the test case got failed this string message got displayed as an error message.
Below examples illustrate the  assertNull() function in PHPUnit:
Example 1:
PHP
<?php use PHPUnit\Framework\TestCase;     class GeeksPhpunitTestCase extends TestCase {     public function testNegativeTestcaseForassertNull()     {         $variable = "Null";             // Assert function to test whether given         // variable is Null or not         $this->assertNull(             $variable,             "variable is null or not"        );     } }     ?> |
Output:
PHPUnit 8.5.8 by Sebastian Bergmann and contributors. F 1 / 1 (100%) Time: 88 ms, Memory: 10.00 MB There was 1 failure: 1) GeeksPhpunitTestCase::testPositiveTestcaseForassertNull variable is null or not Failed asserting that 'Null' is null. /home/lovely/Documents/php/test.php:15 FAILURES! Tests: 1, Assertions: 1, Failures: 1.
Example 2:
PHP
<?php use PHPUnit\Framework\TestCase;     class GeeksPhpunitTestCase extends TestCase {     public function testPositiveTestcaseForassertNull()     {         $variable = Null;             // Assert function to test whether given         // variable is Null or not         $this->assertNull(             $variable,             "variable is null or not"        );     } }     ?> |
Output:
PHPUnit 8.5.8 by Sebastian Bergmann and contributors. . 1 / 1 (100%) Time: 88 ms, Memory: 10.00 MB OK (1 test, 1 assertion)
Reference:https://phpunit.readthedocs.io/en/9.2/assertions.html#assertnull
