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.
There are several stages for which encoding is important. Assuming that you have a string $data, which contains the string you want to pass on in a non-encoded way, these are the relevant stages:
HTML interpretation. In order to specify a random string, you must include it in double quotes, and htmlspecialchars() the whole value.
URL: A URL consists of several parts. If you want your data to be interpreted as one item, you must encode it with urlencode().
Example #1 A hidden HTML form element
echo '<input type="hidden" value="' . htmlspecialchars($data) . '" />'."\n";
Note: It is wrong to urlencode() $data, because it's the browsers responsibility to urlencode() the data. All popular browsers do that correctly. Note that this will happen regardless of the method (i.e., GET or POST). You'll only notice this in case of GET request though, because POST requests are usually hidden.
Example #2 Data to be edited by the user
echo "<textarea name='mydata'>\n";
Note: The data is shown in the browser as intended, because the browser will interpret the HTML escaped symbols. Upon submitting, either via GET or POST, the data will be urlencoded by the browser for transferring, and directly urldecoded by PHP. So in the end, you don't need to do any urlencoding/urldecoding yourself, everything is handled automagically.
Example #3 In a URL
echo '<a href="' . htmlspecialchars("/nextpage.php?stage=23&data=" .
urlencode($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 urlencode()d the data. You'll notice that the & in the URL is replaced by &. 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.
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" />
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'].
Spaces in request variable names are converted to underscores.
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" />
<input name="MyArray" /> <input name="MyArray" /> <input name="MyOtherArray" /> <input name="MyOtherArray" />
<input name="AnotherArray" /> <input name="AnotherArray" /> <input name="AnotherArray[email]" /> <input name="AnotherArray[phone]" />
Specifying array keys 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.
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">