PHP's biggest selling point is often its biggest disadvantage: it's very easy-going on syntax, which often results in people writing poor, unmaintainable code that hasn't been thought out very well if at all. Furthermore, people often use varying programming styles throughout code in a project, and, worse, use strange or outright crazy variable names. A sensible coding style allows you to read code much faster, and also better understand what it was intended to do. It should make different parts of code easily distinguishable.
By adopting a smart coding style, you make an agreement with yourself and fellow programmers on your team to use the same style of variable and function naming, spacing, bracing, and other such key information - your scripts will still work the same, they will just look more similar. When using the same coding style, two independent programmers should be able to create code that is indistinguishable from the code the other created.
PHP, thanks to PEAR, has a very stringently defined "official coding standard". The text in this section is based upon this coding standards - you are free to follow all or part of it if you wish, but either way I recommend you do at least have some coding standard.