It is recommended that you follow the same coding standard while developing a software. The coding standard will allow us easily maintain the code and allow others developer easily read your code. While searching for coding standard for PHP. I found the following sites are good to follow:
After reading the above documents, I found the standard for one tab indentation is 4 spaces. For me, 4 spaces is too narrow and 8 spaces is more readable, therefore I would stick with 8 spaces instead.
Updated on 4/4/2012: Added good post by DerEuroMark
Latest: I now follow PSR-2 coding standard, 4 spaces for indenting. See more details here.