All your code should be properly indented. This is the most fundamental thing you can do to improve readability. Even if you don't comment your code, indenting will be a big help to anyone who has to read your code after you.

while ($x < $z) {
	if ($a == 1) {
		echo 'A was equal to 1';
	} else {
		if ($b == 2) {
			//do something
		} else {
			//do something else
		}
	}
}

The PEAR RFC standard calls for 4 spaces, not tabs of any size, in your code. I disagree with this personally and will continue to tab my code. My view is that tabs rather than spaces will create smaller files and smaller files are faster to parse, upload, download, etc etc. The other advantage to using tabs is that you can set your tab size to your personal preference when viewing someone else's code. I used to use 8-space tabs, but recently switched to 4-space tabs and all my code "reformatted" automatically by just setting a preference in vim.

This is pretty much common sense, but I still see an awful lot of code that is not braced for readability. If you use conditional expressions (IF statements) without braces, not only is it less readable, but bugs can also be introduced when someone modifies your code.

if ($a == 1) echo 'A was equal to 1';

That's pretty much illegible. It may work for you, but the person following after you won't appreciate it at all.

if ($a == 1) 
	echo 'A was equal to 1';

Now that's at least readable, but it's still not maintainable. What if I want an additional action to occur when $a==1? I need to add braces, and if I forget to, I'll have a bug in my code.

if (($a == 1) && ($b==2)) {
	echo 'A was equal to 1';
	//easily add more code
} elseif (($a == 1) && ($b==3)) {
	//do something
}

Notice the space after the if and elseif - this helps distinguish conditionals from function calls.