In PHP, the & symbol is used to reference a variable. It is often used in function calls to pass variables by reference, rather than by value.

When a variable is passed by reference, any changes made to the variable within the function are reflected in the original variable outside of the function. This can be useful for modifying the value of a variable in a function, or for avoiding the overhead of copying large variables.

Here is an example of using the & symbol to pass a variable by reference in PHP:

function increment(&$number) {
$number++;
}

$x = 1;
increment($x);
echo $x; // Outputs 2

In this example, the increment() function increments the value of the $number variable by 1. The & symbol before the $number parameter indicates that the variable is being passed by reference, rather than by value. When the increment() function is called, the value of the $x variable is passed to the $number parameter, and the value of $x is incremented by 1.

The & symbol can also be used in other contexts in PHP, such as when assigning the value of one variable to another using the =& operator:

$x = 1;
$y =& $x;
$y++;
echo $x; // Outputs 2

In this example, the $y variable is assigned to the $x variable using the =& operator. This creates a reference to the $x variable, rather than creating a new copy of the value. Any changes to the $y variable will also affect the $x variable, as they both refer to the same value.

The & symbol can be a useful tool for optimizing your PHP code and for modifying variables in functions.