Don't. Edsger Dijkstra once wrote:
The use of COBOL cripples the mind; its teaching should, therefore, be regarded as a criminal offence.
I'm not sure I'd go quite that far in describing PHP since it actually does have most of the facilities of a modern programming language. I will say that it isn't a good choice for learning the basics of programming, for reasons that are more cultural than technical.
A large amount of the example code that can be found around the net in PHP is less than exemplary relative to what's typical for other languages. To make matters worse, the language itself has a large number of inconsistencies and bad practices in its design. It feels like something that has built up over the years a hack at a time with no coherent design philosophy behind it because, if I understand the history right, that's about how it happened. While many real-world systems are like that, it should be your goal as a programmer to avoid it.
What do you want to get out of learning PHP? Maybe we can suggest some alternatives.