FAQ
PHP Manual

PHP and HTML

PHP and HTML interact a lot: PHP can generate HTML, and HTML can pass information to PHP. Before reading these faqs, it's important you learn how to retrieve variables from external sources. The manual page on this topic includes many examples as well. Pay close attention to what register_globals means to you too.

  1. What encoding/decoding do I need when I pass a value through a form/URL?
  2. I'm trying to use an <input type="image"> tag, but the $foo.x and $foo.y variables aren't available. $_GET['foo.x'] isn't existing either. Where are they?
  3. How do I create arrays in a HTML <form>?
  4. How do I get all the results from a select multiple HTML tag?
  5. How can I pass a variable from Javascript to PHP?
What encoding/decoding do I need when I pass a value through a form/URL?

There are several stages for which encoding is important. Assuming that you have a string $data, which contains the string you want to pass on in a non-encoded way, these are the relevant stages:

  • HTML interpretation. In order to specify a random string, you must include it in double quotes, and htmlspecialchars() the whole value.

  • URL: A URL consists of several parts. If you want your data to be interpreted as one item, you must encode it with urlencode().

Example #1 A hidden HTML form element

<?php
    echo "<input type='hidden' value='" . htmlspecialchars($data) . "' />\n";
?>

Note: It is wrong to urlencode() $data, because it's the browsers responsibility to urlencode() the data. All popular browsers do that correctly. Note that this will happen regardless of the method (i.e., GET or POST). You'll only notice this in case of GET request though, because POST requests are usually hidden.

Example #2 Data to be edited by the user

<?php
    echo "<textarea name='mydata'>\n";
    echo htmlspecialchars($data)."\n";
    echo "</textarea>";
?>

Note: The data is shown in the browser as intended, because the browser will interpret the HTML escaped symbols. Upon submitting, either via GET or POST, the data will be urlencoded by the browser for transferring, and directly urldecoded by PHP. So in the end, you don't need to do any urlencoding/urldecoding yourself, everything is handled automagically.

Example #3 In a URL

<?php
    echo "<a href='" . htmlspecialchars("/nextpage.php?stage=23&data=" .
        urlencode($data)) . "'>\n";
?>

Note: In fact you are faking a HTML GET request, therefore it's necessary to manually urlencode() the data.

Note: You need to htmlspecialchars() the whole URL, because the URL occurs as value of an HTML-attribute. In this case, the browser will first un-htmlspecialchars() the value, and then pass the URL on. PHP will understand the URL correctly, because you urlencoded() the data. You'll notice that the & in the URL is replaced by &amp;. Although most browsers will recover if you forget this, this isn't always possible. So even if your URL is not dynamic, you need to htmlspecialchars() the URL.

I'm trying to use an <input type="image"> tag, but the $foo.x and $foo.y variables aren't available. $_GET['foo.x'] isn't existing either. Where are they?

When submitting a form, it is possible to use an image instead of the standard submit button with a tag like: 

<input type="image" src="image.gif" name="foo" />
When the user clicks somewhere on the image, the accompanying form will be transmitted to the server with two additional variables: foo.x and foo.y.

Because foo.x and foo.y would make invalid variable names in PHP, they are automagically converted to foo_x and foo_y. That is, the periods are replaced with underscores. So, you'd access these variables like any other described within the section on retrieving variables from external sources. For example, $_GET['foo_x'].

Note:

Spaces in request variable names are converted to underscores.

How do I create arrays in a HTML <form>?

To get your <form> result sent as an array to your PHP script you name the <input>, <select> or <textarea> elements like this: 

<input name="MyArray[]" />
<input name="MyArray[]" />
<input name="MyArray[]" />
<input name="MyArray[]" />
Notice the square brackets after the variable name, that's what makes it an array. You can group the elements into different arrays by assigning the same name to different elements: 
<input name="MyArray[]" />
<input name="MyArray[]" />
<input name="MyOtherArray[]" />
<input name="MyOtherArray[]" />
This produces two arrays, MyArray and MyOtherArray, that gets sent to the PHP script. It's also possible to assign specific keys to your arrays: 
<input name="AnotherArray[]" />
<input name="AnotherArray[]" />
<input name="AnotherArray[email]" />
<input name="AnotherArray[phone]" />
The AnotherArray array will now contain the keys 0, 1, email and phone.

Note:

Specifying an arrays key is optional in HTML. If you do not specify the keys, the array gets filled in the order the elements appear in the form. Our first example will contain keys 0, 1, 2 and 3.

See also Array Functions and Variables From External Sources.

How do I get all the results from a select multiple HTML tag?

The select multiple tag in an HTML construct allows users to select multiple items from a list. These items are then passed to the action handler for the form. The problem is that they are all passed with the same widget name. I.e. 

<select name="var" multiple="yes">
Each selected option will arrive at the action handler as: 
var=option1
var=option2
var=option3
Each option will overwrite the contents of the previous $var variable. The solution is to use PHP's "array from form element" feature. The following should be used: 
<select name="var[]" multiple="yes">
This tells PHP to treat $var as an array and each assignment of a value to var[] adds an item to the array. The first item becomes $var[0], the next $var[1], etc. The count() function can be used to determine how many options were selected, and the sort() function can be used to sort the option array if necessary.

Note that if you are using JavaScript the [] on the element name might cause you problems when you try to refer to the element by name. Use it's numerical form element ID instead, or enclose the variable name in single quotes and use that as the index to the elements array, for example: 

variable = documents.forms[0].elements['var[]'];

How can I pass a variable from Javascript to PHP?

Since Javascript is (usually) a client-side technology, and PHP is (usually) a server-side technology, and since HTTP is a "stateless" protocol, the two languages cannot directly share variables.

It is, however, possible to pass variables between the two. One way of accomplishing this is to generate Javascript code with PHP, and have the browser refresh itself, passing specific variables back to the PHP script. The example below shows precisely how to do this -- it allows PHP code to capture screen height and width, something that is normally only possible on the client side.

Example #4 Generating Javascript with PHP

<?php
if (isset($_GET['width']) AND isset($_GET['height'])) {
  // output the geometry variables
  echo "Screen width is: ". $_GET['width'] ."<br />\n";
  echo "Screen height is: ". $_GET['height'] ."<br />\n";
} else {
  // pass the geometry variables
  // (preserve the original query string
  //   -- post variables will need to handled differently)

  echo "<script language='javascript'>\n";
  echo "  location.href=\"${_SERVER['SCRIPT_NAME']}?${_SERVER['QUERY_STRING']}"
            . "&width=\" + screen.width + \"&height=\" + screen.height;\n";
  echo "</script>\n";
  exit();
}
?>

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