Webmaster in a Nutshell, Second Edition

Functions

A function is a named sequence of code statements that can optionally accept parameters and return a value. A function call is an expression that has a value; its value is the returned value from the function. PHP provides a large number of internal functions. The section “Function Reference” lists all commonly available functions. PHP also supports user-definable functions. To define a function, use the function keyword. For example:

function soundcheck($a, $b, $c) {
  return "Testing, $a, $b, $c";
}

When you define a function, you need to be careful what name you give it. In particular, you need to make sure that the name does not conflict with any of the internal PHP functions. If you do use a function name that conflicts with an internal function, you get the following error:

Fatal error: Can't redeclare already declared function in e on line X

After you define a function, you call it by passing in the appropriate arguments. For example:

echo soundcheck(4, 5, 6);

You can also create functions with optional parameters. To do so, set a default value for each optional parameter in the definition, using C++ style. For example, here’s how to make all the parameters to the soundcheck() function optional:

function soundcheck($a=1, $b=2, $c=3) {
  return "Testing, $a, $b, $c";
}

Variable Scope

The scope of a variable refers to where in a program the variable is available. If a variable is defined in the main part of a PHP script (i.e., not inside a function or a class), it is in the ...