How do I assert the result is an integer in PHPUnit?

$this->assertInternalType("int", $id);

Edit: As of PHPUnit 8, the answer is:

$this->assertIsInt($id);

I prefer using the official PHPUnit class constants.

PHPUnit v5.2:

use PHPUnit_Framework_Constraint_IsType as PHPUnit_IsType;

// ...

$this->assertInternalType(PHPUnit_IsType::TYPE_INT, $new_id);

Or latest which at the moment of writing is v7.0:

use PHPUnit\Framework\Constraint\IsType;

// ...

$this->assertInternalType(IsType::TYPE_INT, $new_id);