What are common reasons for the PHP error "Notice: Undefined variable" and how can it be resolved?

The "Notice: Undefined variable" error in PHP occurs when a variable is used without being defined or initialized. This can happen if a variable is referenced before it has been assigned a value. To resolve this issue, make sure to initialize variables before using them in your code.

// Example of resolving "Notice: Undefined variable" error
$variable = ""; // Initialize the variable before using it

echo $variable; // This will not throw an error now