How can you determine the number of occurrences of a keyword in a web page?

I'm trying to figure out how search for the number of occurrence for a keyword on a php webpage. Not a string but count the occurrences and then post the number, for how many times a certain number appears on the page. Are there any functions for this.

The only related function I see is "substr_count". Again I don't want to count the occurrences in a string but a whole page.