Optimizing Execution Time

avoid recomputing values inside a loop

don't do this:

for($i=0; $i < count($array); $i++) {/* do something */}

instead, do this:

$num = count($array);
for($i=0; $i < $num; $i++) { /* do something */ }

include only files that you need; split the functions into more files if they are not always used together
if using a database, use persistent connections
don't use a regular expression when a simple string-manipulation function will do the job; for example, to turn one character into another in a string, use str_replace(), not preg_replace().