Why is the error?

Question

<?php
$name = 'David';
function getName() {
echo $name;
}
getName();

// Error: Undefined variable: name
?>

Answer 1

It's because variables can have different scope. In your case in the function, variable name is not defined, here name is local function scope variable. You can see here a possible solution with global scope variables => accessible from anywhere.
https://code.sololearn.com/wDf0Sxr3HGeq/?ref=app

Answer 2

check this:
http://php.net/manual/en/language.variables.scope.php