The "rule" I follow is this:
If the "if" statement is testing in order to do something (I.E. call functions, configure variables etc.), use braces.
if($test)
{
doSomething();
}
This is because I feel you need to make it clear what functions are being called and where the flow of the program is going, under what conditions. Having the programmer understand exactly what functions are called and what variables are set in this condition is important to helping them understand exactly what your program is doing.
If the "if" statement is testing in order to stop doing something (I.E. flow control within a loop or function), use a single line.
if($test) continue;
if($test) break;
if($test) return;
In this case, what's important to the programmer is discovering quickly what the exceptional cases are where you don't want the code to run, and that is all coverred in $test, not in the execution block.