What are the common reasons for receiving "Notice: Undefined variable" errors in PHP scripts?

The "Notice: Undefined variable" error occurs in PHP scripts when a variable is used without being defined or initialized beforehand. This can happen if a variable is misspelled, not declared within the current scope, or not assigned a value before being used. To solve this issue, make sure to properly define and initialize all variables before using them in your code. Example PHP code snippet:

// Define and initialize the variable before using it
$variable = "Hello World";

// Now the variable can be used without causing an "Undefined variable" error
echo $variable;