I'm very anal about laying out of code. Pretty much all strings are wrapped in single quotes; concatenating strings and variables with single spaces between the string and variable, and variable and string (or two variables, or whatever). Basically:
// Me likey $string = 'This string is ' . $colour;
// Me no likey $string = 'This string is '.$colour; $string = 'This string is '. $colour; $string = 'This string is ' .$colour; $string = "This string is $colour";
The latter (with commas) forgoes the overhead of first concatenating the elements together into one long (in relative terms) string then echoing it, in favour of outputting a series of smaller strings: the upshot being that it is marginally faster to process.