There are two ways to assign one variable to another:

• By value—this creates two variables that hold copies of the same value but remain independent entities.
• By reference—this creates two variable names (aliases) which point to the same space in memory. They cannot be modified separately!

This remains true when dealing with array variables:

$favorites = ["food"=>"pizza", "person"=>"myself", "dog"=>"Tadpole"];
$copy = $favorites;
$alias =& $favorites;
$favorites["food"] = "NEW!";

echo $favorites["food"]; // Prints: NEW!
echo $copy["food"]; // Prints: pizza
echo $alias["food"]; // Prints: NEW!

When passing arrays into functions, both built-in functions and those we write ourselves, we’ll want to be conscious of whether the arrays are being passed by value or by reference.

function changeColor ($arr) 
{
  $arr["color"] = "red";    
}
$object = ["shape"=>"square", "size"=>"small", "color"=>"green"];
changeColor ($object);
echo $object["color"]; // Prints: green

Our function above doesn’t accept its array argument by reference. Therefore, $arr is merely assigned a copy of the argument’s value. This copy array is changed when the function is invoked, but that doesn’t affect the original argument array ($object). To do that, we’d need to pass it by reference:

function reallyChangeColor (&$arr) 
{
  $arr["color"] = "red";    
}
$object = ["shape"=>"square", "size"=>"small", "color"=>"green"];
reallyChangeColor ($object);
echo $object["color"]; // Prints: red

Cool! Let’s write some array functions!