While in PHP mode, you can mark certain parts of your code as a comment that should not be executed. PHP has three ways of doing this: //, /* */, and #. // and # mean, "ignore the rest of this line", whereas /* means "ignore everything until you see */". Some complications exist with /* and */, though, which make them less desirable to use.
<?php print"This is printed\n"; // print "This is not printed\n";
# print "This is not printed\n"; print"This is printed\n"; /* print "This is not printed\n";
print "This is not printed\n"; */ ?>
That chunk of code shows all three types of comments in action, but does not demonstrate the problem with the /* */ form of commenting. If you were to start a /* comment on line one, and end it on the line near the bottom where the other /* comment is started, you would find that the script would fail to work. The reason for this is that you cannot stack up, or "nest" /* */ comments, and attempting to do so will usually fail spectacularly.
It is generally best to stick to // for your commenting purposes, simply because it is easier to spot, easy to read, and easy to keep control of.