PHP - Notice: Undefined variable error

The Mistake Undefined variable often appears during PHP script execution as a message:

Notice: Undefined variable: nom_variable in C:\wamp\www\project\index.php on line 14

ca what does this error mean and how do I fix it?

The Undefined variable error is caused because it is:
- It has not been declared in code.
- It is used in another file that uses the same variable name.
- It has not been initialized.

The isset() detects if the variable exists and initializes. Again, the method empty() is more optimal because it doesn't generate an error message.

Example:

foreach ($array as $item) {
 // do something
 $count++; 
}

The count variable was not set, and in this case the error  Undefined variable  appears.
To fix this, add the count variable before the loop without forgetting the initialization to zero.

$count=0; 
foreach ($array as $item) {
 // do something
 $count++; 
}

Example 2:

session_start(); 
// recommended solution
$nom = $_SESSION['nom_utilisateur']; 
if (empty($nom_user)) $nom_user = ''; 

Or

// set the variable at the beginning of index.php
$nom = ''; 
$nom = $_SESSION['nom_utilisateur']; 

Or 

$nom = $_SESSION['nom_utilisateur']; 
if (!isset($nom_user)) $nom_user = ''; 

Don't forget to initialize the session with session_start()  otherwise the server will not be able to identify and read the variables $_SESSION.

Resources:
http://php.net/manual/en/function.empty.php
PHP: "Notice: Undefined variable"