PHPUnit - Use $this or self for static methods?

The phpunit documentation says you can use either and doesnt advocate one over the other. So you choose! https://phpunit.readthedocs.io/en/9.2/assertions.html

PHPUnit 4.8.9: vendor/phpunit/phpunit/src/Framework/Assert.php:

/**
 * Asserts that a condition is true.
 *
 * @param bool   $condition
 * @param string $message
 *
 * @throws PHPUnit_Framework_AssertionFailedError
 */
public static function assertTrue($condition, $message = '')
{
    self::assertThat($condition, self::isTrue(), $message);
}

Technically static::assertTrue() is correct, but the common usage of the assert methods is $this->assertTrue().

Generally, self is only used to refer to static methods and properties (though confusingly you can refer to non-static methods with self, and to static methods with $this, provided the methods called with self don't reference $this.)

<?php
class Test {
    public static function staticFunc() {echo "static ";}
    public function nonStaticFunc() {echo "non-static\n";}
    public function selfCaller() {self::staticFunc(); self::nonStaticFunc();}
    public function thisCaller() {$this->staticFunc(); $this->nonStaticFunc();}
}
$t = new Test;
$t->selfCaller();  // returns "static non-static"
$t->thisCaller();  // also returns "static non-static"

Inheritance is important to remember when dealing with $this or self. $this will always refer to the current object, while self refers to the class in which self was used. Modern PHP also includes late static binding via the static keyword, which will operates the same way as (and should be preferred over) $this for static functions.

<?php
class Person {
    public static function whatAmI() {return "Human";}    
    public function saySelf() {printf("I am %s\n", self::whatAmI());}
    public function sayThis() {printf("I am %s\n", $this->whatAmI());}
    public function sayStatic() {printf("I am %s\n", static::whatAmI());}
}

class Male extends Person {
    public static function whatAmI() {return "Male";}
}

$p = new Male;
$p->saySelf();    // returns "I am Human"
$p->sayThis();    // returns "I am Male"
$p->sayStatic();  // returns "I am Male"

As regards PHPUnit in particular, it appears they simply do things the way they've always done them! Though according to their documentation, your code should work fine using static methods.