PHPUnitで定数のテストをするときは@runInSeparateProcessで別プロセスに ref: http://qiita.com/items/1476

file0.php

<?php
class ConstantTest extends PHPUnit_Framework_TestCase
{
    /**
     * @runInSeparateProcess
     */
    public function testConstant1()
    {
        define('THIS_IS_CONSTANT', 'foo');
        $this->assertSame('foo', THIS_IS_CONSTANT);
    }

    /**
     * @runInSeparateProcess
     */
    public function testConstant2()
    {
        define('THIS_IS_CONSTANT', 'bar');
        $this->assertSame('bar', THIS_IS_CONSTANT);
    }

    /**
     * @runInSeparateProcess
     */
    public function testConstant3()
    {
        define('THIS_IS_CONSTANT', 'baz');
        $this->assertSame('baz', THIS_IS_CONSTANT);
    }
}

file1.txt

PHPUnit 3.6.3 by Sebastian Bergmann.

...

Time: 0 seconds, Memory: 4.75Mb

OK (3 tests, 3 assertions)