Method chaining is a programming technique that allows you to call multiple methods on the same object in a single statement. Instead of calling one method at a time and assigning the result to a variable, you can chain method calls together. This can lead to more concise and readable code.
In PHP, method chaining is often facilitated by designing methods in a way that they return the current object ($this) after performing their operations. This allows you to call another method on the same object without needing to store the object in a variable between method calls.

public function setValue($value) {
    $this->value = $value;
    return $this; // Return the current object
}

public function multiplyBy($factor) {
    $this->value *= $factor;
    return $this; // Return the current object
}

public function getResult() {
    return $this->value;
}

echo $result; // Outputs 15

.vendor/bin/phpunit --filter [test method name] ex : ./vendor/bin/phpunit test_no_routes_when_created

$this->router = new Router(); vs $router = new Router();

$this : refers to current instance of class while $router is a variable. $this->router is an instance variable (property) of the class, and assigning a new instance of Router to it means you are setting the value of the class property.
$router : This syntax is used outside the context of a class or within a function that is not part of a class. $router is a local variable within the current scope (function, method, or global scope, depending on where it's used). It doesn't have the same visibility as a class property.