Use [code = php]
[/code]
Without a space on either side of = .

Used to do the same before Peter_Budo told me about the noparse tags.

You can use the noparse tags around your code block like this :-

[noparse]
[code=xyz]
[/code]

[/noparse]

You can read more about the various vB tags here.

Thanks! I didn't know about [noparse] tags.

The only problem with explaining the usage of noparse tags is that it becomes somewhat cumbersome to explain the exact usage of the noparse tags themselves. For example, consider the following:

The above example was created by using two nested levels of noparse tags. But that's so dull and boring! Some bbcode formatting would help make our theoretical explanation clearer. The problem is of course, you can't put bbcode formatting inside a noparse tag because of the very nature of a noparse tag. Therefore, we have to resort to a little bit of bbcode black magic. Using the fact that [some[b][/b]tag] will be parsed into [sometag], we can quickly see that we can implement the same intended functionality of noparse while still being able to selectively enable the bbcode parser.

Thus, we can write the following in our post:
[no[b][/b]parse] [b][co[b][/b]de=[col[b][/b]or="red"]c[/color]][/b] [inlinecode]code goes here[/inlinecode] [b][/code][/b] [/noparse]

Which parses down nicely to become:
There. Now you can with confidence explain to other people not only how to use code tags, but how to clearly use noparse tags with full bbcode formatting.

I am getting a head ache.

And if you don't like having to type so much to yell at the noobs, the Clippings add-on for Firefox can it effortless