Notice: Undefined variable: my_variable_name in C:\wamp\www\mypath\index.php on line 10

Notice: Undefined index: my_index C:\wamp\www\mypath\index.php on line 11

echo "My variable value is: " . $my_variable_name;
echo "My index value is: " . $my_array["my_index"];

             This is a General Reference question for people to link to as duplicate, instead of having to explain the issue over and over again. I feel this is necessary because most real-world answers on this issue are very specific.

Related Meta discussion:

• What can be done about repetitive questions?
• Do “reference questions” make sense?

Notice: Undefined variable

                 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,"",null.

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

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'] : '';

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.

Notice: Undefined index / Undefined offset

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');

       Notice: Undefined offset: 1

$_POST / $_GET / $_SESSION variable

• Notice: Undefined variable
• Notice: Undefined Index