"Notice: Undefined variable", "Notice: Undefined index", and "Notice: Undefined offset" using PHP

Notice: Undefined variable

From the vast wisdom of the PHP Manual:

Relying on the default value of an uninitialized variable is problematic in the case of including one file into another which uses the same variable name. It is also a major security risk with register_globals turned on. E_NOTICE level error is issued in case of working with uninitialized variables, however not in the case of appending elements to the uninitialized array. isset() language construct can be used to detect if a variable has been already initialized. Additionally and more ideal is the solution of empty() since it does not generate a warning or error message if the variable is not initialized.

From PHP documentation:

No warning is generated if the variable does not exist. That means empty() is essentially the concise equivalent to !isset($var) || $var == false.

This means that you could use only empty() to determine if the variable is set, and in addition it checks the variable against the following, 0, 0.0, "", "0", null, false or [].

Example:

$o = [];
@$var = ["",0,null,1,2,3,$foo,$o['myIndex']];
array_walk($var, function($v) {
    echo (!isset($v) || $v == false) ? 'true ' : 'false';
    echo ' ' . (empty($v) ? 'true' : 'false');
    echo "\n";
});

Test the above snippet in the 3v4l.org online PHP editor

Although PHP does not require a variable declaration, it does recommend it in order to avoid some security vulnerabilities or bugs where one would forget to give a value to a variable that will be used later in the script. What PHP does in the case of undeclared variables is issue a very low level error, E_NOTICE, one that is not even reported by default, but the Manual advises to allow during development.

Ways to deal with the issue:

  1. Recommended: Declare your variables, for example when you try to append a string to an undefined variable. Or use isset() / !empty() to check if they are declared before referencing them, as in:

    //Initializing variable
$value = ""; //Initialization value; Examples
             //"" When you want to append stuff later
             //0  When you want to add numbers later
//isset()
$value = isset($_POST['value']) ? $_POST['value'] : '';
//empty()
$value = !empty($_POST['value']) ? $_POST['value'] : '';

    This has become much cleaner as of PHP 7.0, now you can use the null coalesce operator:

    // Null coalesce operator - No need to explicitly initialize the variable.
$value = $_POST['value'] ?? '';

  2. Set a custom error handler for E_NOTICE and redirect the messages away from the standard output (maybe to a log file): 

    set_error_handler('myHandlerForMinorErrors', E_NOTICE | E_STRICT)

  3. Disable E_NOTICE from reporting. A quick way to exclude just E_NOTICE is:

    error_reporting( error_reporting() & ~E_NOTICE )

  4. Suppress the error with the @ operator.

Note: It's strongly recommended to implement just point 1.

Notice: Undefined index / Undefined offset

This notice appears when you (or PHP) try to access an undefined index of an array.

Ways to deal with the issue:

  1. Check if the index exists before you access it. For this you can use isset() or array_key_exists():

    //isset()
$value = isset($array['my_index']) ? $array['my_index'] : '';
//array_key_exists()
$value = array_key_exists('my_index', $array) ? $array['my_index'] : '';

  2. The language construct list() may generate this when it attempts to access an array index that does not exist:

    list($a, $b) = array(0 => 'a');
//or
list($one, $two) = explode(',', 'test string');

Two variables are used to access two array elements, however there is only one array element, index 0, so this will generate:

Notice: Undefined offset: 1

$_POST / $_GET / $_SESSION variable

The notices above appear often when working with $_POST, $_GET or $_SESSION. For $_POST and $_GET you just have to check if the index exists or not before you use them. For $_SESSION you have to make sure you have the session started with session_start() and that the index also exists.

Also note that all 3 variables are superglobals and are uppercase.

Related:

  • Notice: Undefined variable
  • Notice: Undefined Index

Try these

Q1: this notice means $varname is not defined at current scope of the script.

Q2: Use of isset(), empty() conditions before using any suspicious variable works well.

// recommended solution for recent PHP versions
$user_name = $_SESSION['user_name'] ?? '';

// pre-7 PHP versions
$user_name = '';
if (!empty($_SESSION['user_name'])) {
     $user_name = $_SESSION['user_name'];
}

Or, as a quick and dirty solution: 

// not the best solution, but works
// in your php setting use, it helps hiding site wide notices
error_reporting(E_ALL ^ E_NOTICE);

Note about sessions:

  • When using sessions, session_start(); is required to be placed inside all files using sessions.

  • http://php.net/manual/en/features.sessions.php


Error display @ operator

For undesired and redundant notices, one could use the dedicated @ operator to »hide« undefined variable/index messages.

$var = @($_GET["optional_param"]);
  • This is usually discouraged. Newcomers tend to way overuse it.
  • It's very inappropriate for code deep within the application logic (ignoring undeclared variables where you shouldn't), e.g. for function parameters, or in loops.
  • There's one upside over the isset?: or ?? super-supression however. Notices still can get logged. And one may resurrect @-hidden notices with: set_error_handler("var_dump");
    • Additonally you shouldn't habitually use/recommend if (isset($_POST["shubmit"])) in your initial code.
    • Newcomers won't spot such typos. It just deprives you of PHPs Notices for those very cases. Add @ or isset only after verifying functionality.

    • Fix the cause first. Not the notices.

  • @ is mainly acceptable for $_GET/$_POST input parameters, specifically if they're optional.

And since this covers the majority of such questions, let's expand on the most common causes:

$_GET / $_POST / $_REQUEST undefined input

  • First thing you do when encountering an undefined index/offset, is check for typos:
    $count = $_GET["whatnow?"];

    • Is this an expected key name and present on each page request?
    • Variable names and array indicies are case-sensitive in PHP.

  • Secondly, if the notice doesn't have an obvious cause, use var_dump or print_r to verify all input arrays for their curent content:

    var_dump($_GET);
var_dump($_POST);
//print_r($_REQUEST);

    Both will reveal if your script was invoked with the right or any parameters at all.

  • Alternativey or additionally use your browser devtools (F12) and inspect the network tab for requests and parameters:

    browser developer tools / network tab

    POST parameters and GET input will be be shown separately.

  • For $_GET parameters you can also peek at the QUERY_STRING in

    print_r($_SERVER);

    PHP has some rules to coalesce non-standard parameter names into the superglobals. Apache might do some rewriting as well. You can also look at supplied raw $_COOKIES and other HTTP request headers that way.

  • More obviously look at your browser address bar for GET parameters:

    http://example.org/script.php?id=5&sort=desc

    The name=value pairs after the ? question mark are your query (GET) parameters. Thus this URL could only possibly yield $_GET["id"] and $_GET["sort"].

  • Finally check your <form> and <input> declarations, if you expect a parameter but receive none.

    • Ensure each required input has an <input name=FOO>
    • The id= or title= attribute does not suffice.
    • A method=POST form ought to populate $_POST.
    • Whereas a method=GET (or leaving it out) would yield $_GET variables.
    • It's also possible for a form to supply action=script.php?get=param via $_GET and the remaining method=POST fields in $_POST alongside.
    • With modern PHP configurations (≥ 5.6) it has become feasible (not fashionable) to use $_REQUEST['vars'] again, which mashes GET and POST params.

  • If you are employing mod_rewrite, then you should check both the access.log as well as enable the RewriteLog to figure out absent parameters.

$_FILES

  • The same sanity checks apply to file uploads and $_FILES["formname"].
  • Moreover check for enctype=multipart/form-data
  • As well as method=POST in your <form> declaration.
  • See also: PHP Undefined index error $_FILES?

$_COOKIE

  • The $_COOKIE array is never populated right after setcookie(), but only on any followup HTTP request.
  • Additionally their validity times out, they could be constraint to subdomains or individual paths, and user and browser can just reject or delete them.

Tags:

Php

Arrays

Variables

Warnings

Undefined Index

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