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Looping
One situation in which the use of variables is crucial is in looping (repetitive processing). For example, part of displaying the calendar will be creating a link for each day. Ultimately, these links will be part of a table, but for now we'll create a table that consists of just one row, as shown in Listing 5.
Listing 5—Using a for Loop
<html>
<head><title>Event Calendar</title></head>
<body>
<?php
$numberofdays = 31;
$daysoftheweek = array("SUNDAY", "MONDAY", "TUESDAY",
"WEDNESDAY", "THURSDAY", "FRIDAY",
"SATURDAY");
?>
<h1>This Month's Events</h1>
<table border="1">
<tr>
<?php
for ($i = 1;
$i <= $numberofdays;
$i++)
{
echo("<td>$i</td>");
}
?>
</tr>
</table>
</body>
</html>
Notice that we've interspersed HTML and PHP code. You can include as many PHP sections as you want on a page, but in practice it's a good idea to avoid switching back and forth a lot.
Now let's take a look at the for loop in Listing 5. Within the parentheses are three statements:
|
$i = 1; |
Initializes a new variable, $i, with a value of 1. |
|
$i <= $numberofdays; |
Defines an end state for the loop; processing will continue as long as the value of $i is less than the value of $numberofdays. When that statement is no longer true, the loop exits. |
|
$i++) |
Each time the loop executes, it increments the value of $i by 1. |
The actions performed within the loop itself—in this case, just an echo() statement that outputs the value of $i within a td tag—are contained within the braces ({}).
Putting it all together, the loop starts in a state in which $i equals 1, increments $i by 1 each time it executes, and keeps going as long as $i is less than $numberofdays, outputting the current value each time.
The end result looks like Figure 4.
)
Figure 4 Using a for loop.
This kind of loop is handy when you know exactly how many times you want the loop to execute, and when the index, represented here by $i, is predictable. Sometimes, though, you need more flexibility. For example, you might have an array with non-consecutive index values. In these cases, it's better to use a foreach loop, as shown in Listing 6.
Listing 6—Using a foreach Loop
...
$daysoftheweek = array("SUNDAY", "MONDAY", "TUESDAY",
"WEDNESDAY", "THURSDAY", "FRIDAY",
"SATURDAY");
?>
<h1>This Month's Events</h1>
<table border="1">
<tr>
<?php
foreach ($daysoftheweek as $thisday) {
echo("<th>$thisday</th>");
}
?>
</tr>
<tr>
<?php
for ($i = 1; $i <= $numberofdays; $i++) {
...
A foreach loop executes once for each value within a group. For example, Listing 6 shows a loop that runs through the $daysoftheweek array. Each time the loop is processed, the next value within the array is assigned to $thisday, and the statements are executed. When there are no more values, the loop terminates. This way, we output each of the values within the array, as shown in Figure 5.
)
Figure 5 Using a foreach loop.
Of course, this is hardly the way we expect a calendar to look! To get the layout right, we need to make two adjustments: Show the first day of the month as the correct day of the week, and create a new row for each week. Both of these tasks require functions.