Top Ten Tips for Getting Started with PHP

Last but not least, all PHP functions must return acceptable data to the code that called them. The tricky part of this apparently superfluous statement is the fact that the meaning of acceptable depends on the whole script, and it may be different at any time. Here is a very dumb, but effective example of what we mean:

function subtraction($A, $B) {
	$diff = $A - $B;
  	return($diff);
}
   $C = 1/subtraction(3, 3);      // ERROR! Division by Zero!
   $D = 1/(1 - subtraction(3,3);

Although calculating $C will make the script crash, calculating (with the same operands), $D will not. The point is that before doing anything with a variable, you should check that it has an acceptable value. In the example above, this would mean assigning the subtraction result to an auxiliary variable and proceeding with the division only if it is non-null.

It is even more important to check return values from system calls, that is, the built-in functions provided to allow interaction with external processes and files. Should you forget to check a return value, data could be thrown away without anyone noticing, as in this example:

<?php
$HANDLE = fopen("newuser.txt","w")); // open a file
fwrite($HANDLE, "New User Data");    // write to it
?>

If fopen fails (because, for example, the disc is full or you had no permission to write) the New User Data is lost for good. Before writing, check that $HANDLE is not null:

<?php
if (!$HANDLE = fopen("newuser.txt","w")) { die "File access failed: newuser.txt"; }
fwrite($HANDLE, "New User Data");
?>

Happy PHP coding!