There are times when you want to take information contained in a Web page, and pass it to another Web page without requiring any input from the user. There's another setting for the <input> control that enables you to pass information in a field just as if it were a text box but keeping the control and its contents hidden. This is known as a hidden form field (or hidden control).
Hidden form fields come into play in a slightly different manner than the controls already demonstrated. They're probably more useful on PHP pages that contain forms because you can use them to send information contained within PHP variables. Here's a typical hidden form field on a form:
<input type="hidden" name="Hidden1" value="Secret Message">
There's no screenshot of this because the control wouldn't appear on the page. Any form that submitted it though would have a variable called $Hidden1 that contains the text Secret Message. To use the hidden form field in a PHP page, you can write the whole HTML form in echo() statements, transferring the contents of PHP variables via HTML controls as shown here:
<?php $Message1="This message is invisible"; echo "<form>"; echo "<input type='hidden' name='Hidden2' value='$Message1'>"; echo "<input type='submit' value='Submit'>"; echo "</form>"; ?>
This entire HTML form is written in PHP statements and it enables you to create a variable called $Hidden2 and transfer the contents of $Message1 into it. Of course, this is not the only way to keep data available across form submissions or page requests; cookies, sessions, and so on are discussed later on in this chapter.
Now for an example that takes the contents of a <select> list box and displays the user's choice on the next page. The preceding process is used to write the HTML form in PHP echo statements as well.
Try It Out: Use the Hidden Form Field
Once you get over the process of creating the HTML form in echo statements rather than straight HTML code, this is really very straightforward. The main difference is that you can leave out the apostrophes in the variable name in the array. Three variables are created to form the basis of the <select> list box:
$Message1="Bugs Bunny"; $Message2="Homer Simpson"; $Message3="Ren & Stimpy";
They are respectively $Message1, $Message2, and $Message3. Next, the HTML form is created using echo statements. Absolutely nothing differs from a normal HTML form, except that when you want to use quotation marks in the HTML, you have to use single quotation marks and not double ones or you'll break the echo statement. The first line just tells you to send the form contents to hidden2.php via the GET method:
echo "<form method='GET' action='hidden2.php'>";
Some explanatory text is displayed and then the <select> list box is started:
echo "Which of the following would win in a shootout?"; echo "<select name='ListBox'>";
It gets three options, the contents of the variables $Message1, $Message2, and $Message3, respectively.
echo "<option>$Message1</option>"; echo "<option>$Message2</option>"; echo "<option>$Message3</option>";
The <select> list box is closed and a couple of line breaks are added:
Then the three variables already used are passed as hidden form fields to the form:
echo "<input type='hidden' name='Hidden1' value='$Message1'>"; echo "<input type='hidden' name='Hidden2' value='$Message2'>"; echo "<input type='hidden' name='Hidden3' value='$Message3'>";
The three variables will turn up on the form as $Hidden1, $Hidden2, and $Hidden3, respectively. A Submit button is added to the form, and the form is closed:
echo "<input type='submit' value='Submit'>"; echo "</form>";
The second PHP page just displays the contents of the controls created in the first page. The contents of the three hidden form fields are displayed first:
This is useful because normally the contents of the entire list box aren't transferred across. Only the option selected by the user is passed over to the next PHP page. However, sometimes you want to have all of the list box contents available in your PHP page. This is one effective method for transferring this type of information.
The last lines just display the contents of selection made by the user.
echo "<br>You selected:<br>"; echo "$_GET[ListBox]";
Hidden form fields are used to perform this type of task later in this book.
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