I'm beginning to learn PHP but am told that I should have a 'proficient' grasp of HTML before I begin.

Forgetting how much HTML I must know to learn PHP, what would you call 'knowing enough' HTML?

I've only run through a few tutorials in what I imagine are the basics.

I suppose this is a difficult question to pin point an answer, but what is the upper boundry of knowledge any person who 'knows HTML' should have?