How to: Pass variable to another page

HTML / HTTP is stateless, in other words, what you did / saw on the previous page, is completely disconnected with the current page. Except if you use something like sessions, cookies or GET / POST variables. Sessions and cookies are quite easy to use, with session being by far more secure than cookies. More secure, but not completely secure.

Session:

//On page 1
$_SESSION['varname'] = $var_value;

//On page 2
$var_value = $_SESSION['varname'];

Remember to run the session_start() statement on both these pages before you try to access the $_SESSION array, and also before any output is sent to the browser.

Cookie:

//One page 1
$_COOKIE['varname'] = $var_value;

//On page 2
$var_value = $_COOKIE['varname'];

The big difference between sessions and cookies are that the value of the variable will be stored on the server if you're using sessions, and on the client if you're using cookies. I can't think of any good reason to use cookies instead of sessions, except if you want data to persist between sessions, but even then it's perhaps better to store it in a DB, and retrieve it based on a username or id.

GET and POST

You can either add the variable in the link to the next page:

<a href="page2.php?varname=<?php echo $var_value ?>">Page2</a>

This will create a GET variable, or include a hidden field in a form that submits to page two:

<form method="get" action="page2.php">
    <input type="hidden" name="varname" value="var_value">
    <input type="submit">
</form>

And then on page two

//Using GET
$var_value = $_GET['varname'];

//Using POST
$var_value = $_POST['varname'];

//Using GET, POST or COOKIE.
$var_value = $_REQUEST['varname'];

Just change the method for the form to post if you want to do it via post. Both are equally insecure, although GET is easier to hack.

The fact that each new request is, except for session data, a totally new instance of the script caught me when I first started coding in PHP. Once you get used to it, it's quite simple though.

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Example PHP: cURL HTTP POST

You'll find php/curl examples here: http://curl.haxx.se/libcurl/php/examples/, especially http://curl.haxx.se/libcurl/php/examples/simplepost.html

<?php
//
// A very simple PHP example that sends a HTTP POST to a remote site
//

$ch = curl_init();

curl_setopt($ch, CURLOPT_URL,"http://www.mysite.com/tester.phtml");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,
            "postvar1=value1&postvar2=value2&postvar3=value3");

// in real life you should use something like:
// curl_setopt($ch, CURLOPT_POSTFIELDS, 
//          http_build_query(array('postvar1' => 'value1')));

// receive server response ...
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

$server_output = curl_exec ($ch);

curl_close ($ch);

// further processing ....
if ($server_output == "OK") { ... } else { ... }

?>

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Post data using file_get_contents in PHP

Sending an HTTP POST request using file_get_contents is not that hard, actually : as you guessed, you have to use the $context parameter.

There's an example given in the PHP manual, at this page : HTTP context options (quoting) :

$postdata = http_build_query(
    array(
        'var1' => 'some content',
        'var2' => 'doh'
    )
);

$opts = array('http' =>
    array(
        'method'  => 'POST',
        'header'  => 'Content-type: application/x-www-form-urlencoded',
        'content' => $postdata
    )
);

$context  = stream_context_create($opts);

$result = file_get_contents('http://example.com/submit.php', false, $context);

Basically, you have to create a stream, with the right options (there is a full list on that page), and use it as the third parameter to file_get_contents -- nothing more

As a sidenote : generally speaking, to send HTTP POST requests, we tend to use curl, which provides a lot of options an all -- but streams are one of the nice things of PHP that nobody knows about... too bad...

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Example: jQuery AJAX POST with PHP

Basic usage of .ajax would look something like this:

HTML:

<form id="foo">

    <label for="bar">A bar</label>
    <input id="bar" name="bar" type="text" value="" />

    <input type="submit" value="Send" />

</form>

JavaScript:

// variable to hold request
var request;
// bind to the submit event of our form
$("#foo").submit(function(event){
    // abort any pending request
    if (request) {
        request.abort();
    }
    // setup some local variables
    var $form = $(this);
    // let's select and cache all the fields
    var $inputs = $form.find("input, select, button, textarea");
    // serialize the data in the form
    var serializedData = $form.serialize();

    // let's disable the inputs for the duration of the ajax request
    // Note: we disable elements AFTER the form data has been serialized.
    // Disabled form elements will not be serialized.
    $inputs.prop("disabled", true);

    // fire off the request to /form.php
    request = $.ajax({
        url: "/form.php",
        type: "post",
        data: serializedData
    });

    // callback handler that will be called on success
    request.done(function (response, textStatus, jqXHR){
        // log a message to the console
        console.log("Hooray, it worked!");
    });

    // callback handler that will be called on failure
    request.fail(function (jqXHR, textStatus, errorThrown){
        // log the error to the console
        console.error(
            "The following error occured: "+
            textStatus, errorThrown
        );
    });

    // callback handler that will be called regardless
    // if the request failed or succeeded
    request.always(function () {
        // reenable the inputs
        $inputs.prop("disabled", false);
    });

    // prevent default posting of form
    event.preventDefault();
});

Note: Since jQuery 1.8, .success, .error and .complete are deprecated in favor of .done, .fail and .always.

Note: Remember that the above snippet has to be done after DOM ready, so you should put it inside a $(document).ready() handler (or use the $() shorthand).

Tip: You can chain the callback handlers like this: $.ajax().done().fail().always();

PHP (i.e. form.php):

// you can access the values posted by jQuery.ajax
// through the global variable $_POST, like this:
$bar = $_POST['bar'];

Note: Always sanitize posted data, to prevent injections and other malicious code.

You could also use the shorthand .post in place of .ajax in the above JavaScript code:

$.post('/form.php', serializedData, function(response) {
    // log the response to the console
    console.log("Response: "+response);
});

Note: The above JavaScript is made to work with jQuery 1.8 but should work with previous versions down to jQuery 1.5.

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