Demonstration of the speed difference between calling is_null and using a comparison operator.

test_null.php

<?php

function runTestFunction($name, Callable $function) {
    $startTime = microtime(true);
    for ($i = 0; $i < 10000000; $i++) {
        $function();
    }
    $endTime = microtime(true);
    echo $name . ' took ' . ($endTime - $startTime);
    echo PHP_EOL;
}

function testNullWithFunction() {
    $testValue = null;
    is_null($testValue);
}

function testNotNullWithFunction() {
    $testValue = 'a';
    is_null($testValue);
}

function testNullWithoutFunction() {
    $testValue = null;
    $testValue === null;
}

function testNotNullWithoutFunction() {
    $testValue = 'a';
    $testValue === null;
}

runTestFunction('NullWithFunction', 'testNullWithFunction');
runTestFunction('NotNullWithFunction', 'testNotNullWithFunction');
runTestFunction('NullWithComparison', 'testNullWithoutFunction');
runTestFunction('NotNullWithComparison', 'testNotNullWithoutFunction');

// NullWithFunction took 16.489717960358
// NotNullWithFunction took 14.222808837891
// NullWithComparison took 9.0285971164703
// NotNullWithComparison took 9.2056560516357