PHP by itself may not be the best load testing language, as it is (normally) single-threaded. If, for example, you use it to make numerous cURL requests against the site being tested, it will only send one request at a time. For heavier testing involving parallel requests, you might want to look into a language/tool that supports multi-threading and therefore could submit multiple parallel request to the system under test.
A nice little tool I've used in the past and with a good price (free) is Apache JMeter. But, if you are set on doing something in PHP, as mentioned above, start by looking at its cURL extension.
