Calling ReflectionAttribute::newInstance() using an attribute name that does not have a corresponding class will result in an error.

Example :

<?php
#[FakeAttribute]
class Test {}

try {
  (new ReflectionClass(Test::class))->getAttributes()[0]->newInstance();
}
catch (Error $error) {
  echo 'Throwed error ' . get_class($error) . ' with message : ' . $error->getMessage();
}
?>

This will output :

Throwed error Error with message : Attribute class "FakeAttribute" not found