How do I convert datetime to ISO 8601 in PHP?

After PHP 5 you can use this: echo date("c"); form ISO 8601 formatted datetime.

http://ideone.com/nD7piL

Note for comments:

Regarding to this, both of these expressions are valid for timezone, for basic format: ±[hh]:[mm], ±[hh][mm], or ±[hh].

But note that, +0X:00 is correct, and +0X00 is incorrect for extended usage. So it's better to use date("c"). A similar discussion here.