What is the function used to determine the length of a variable in PHP?

To determine the length of a variable in PHP, you can use the built-in function `strlen()`. This function takes a string as an argument and returns the number of characters in that string. It is useful for checking the length of a string variable before performing certain operations or validations.

$myString = "Hello, World!";
$length = strlen($myString);
echo "The length of the string is: " . $length;