We can invoke functions with variables or with values directly. When we invoke a function with a variable as its argument, it’s as if we’re assigning the value held by that variable to the function’s parameter. We assign a copy of the value held by the argument variable. The variable argument and the parameter are distinct entities; changes made inside the function to the parameter will not affect the variable that was passed in:

function addX ($param)
{
  $param = $param . "X";
  echo $param;
};
$word = "Hello";
addX($word); // Prints: HelloX
echo $word; // Prints: Hello

Let’s walk through the code above:

• We defined a function addX() which reassigns $param to its previous value appended with "X".
• We defined the variable $word and assigned it the value "Hello".
• We invoked addX() with $word as its argument.
• During the function invocation, $param was reassigned and the function printed "HelloX".
• When we printed $word after the function was invoked, it remained its original value: "Hello".

If we do want to make permanent changes to a variable within a function, we can prepend the parameter name with the reference sign (&). In this way, we assign the parameter to be an alias for the argument variable. Both will refer to the same spot in memory, and changes to the parameter within the function will permanently affect the argument variable.

function addXPermanently (&$param)
{
  $param = $param . "X";
  echo $param;
};
$word = "Hello";
addXPermanently($word); // Prints: HelloX
echo $word; // Prints: HelloX

In the addXPermanently() function we made $param a reference to the argument. When we invoked the function with $word the changes made to $param permanently affected the $word variable. Let’s practice!