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Functions and Scope
A function doesn't actually have to return a value. Instead, it can take information and use it to perform an action. (In other languages, this might be called a procedure or a subroutine.) For example, we can create a function that simply sets the $month and $year variables based on the values submitted with the form, as shown in Listing 2.
Listing 2—Functions that Don't Return a Value
...
function get_current_month(){
$today = getdate();
return $today['mon'];
}
function get_current_year(){
$today = getdate();
return $today['year'];
}
function set_month ($submittedMonth){
global $month;
if (!isset($submittedMonth)) {
$month = get_current_month();
} else {
if ((intval($submittedMonth) > 0) && (intval($submittedMonth) <= 12)) {
$month = $submittedMonth;
} else {
$month = get_current_month();
}
}
}
function set_year ($submittedYear) {
global $year;
if (!isset($submittedYear)) {
$year = get_current_year();
} else {
if (intval($submittedYear) == $submittedYear) {
$year = $submittedYear;
} else {
$year = get_current_year();
}
}
}
//Initialize variables
import_request_variables('pgc', 'g_');
set_month($g_month);
set_year($g_year);
...
Looking at set_month() in particular, we see that it takes a single argument, $submittedMonth, which is populated with the value of $g_month when the page calls set_month($g_month). The function then tests $submittedMonth to make sure that it's actually set. In other words, was a value of $g_month submitted with the form? And if it is, is it a positive integer less than or equal to 12? If either one of those tests fails, the function sets the value of $month by calling our previous function, get_current_month(). If neither test fails, the function sets the value to $submittedMonth.
In both cases, the function simply sets the value of the $month variable, but that's not quite the end of the story. Each function has its own scope, meaning that a variable declared within the function exists only within that function. For example, $submittedMonth has no value outside the function.
As a side effect of this behavior, if we simply referenced the $month variable within the function, we would set the value not of the main $month variable used by the rest of the application, but of a separate $month variable that exists only within the function.
To combat this, we can use the global keyword to tell the function to use the $month variable from the main application, rather than creating its own version.
Now we're ready to look at adding functions to classes.