[PHP] The reference of function

Assume that we have such function:

function testRef(&$a = array(10, 20, 30))
{
    echo "Test of reference.\n";
}

When we use it as below:

/*
 *
 *Example 1
 *
 */
testRef(array(10, 30, 40));
//The out is :
//Fatal error: Only variables can be passed by reference


/*
 *
 *Example 2
 *
 */
$a = array(10, 30, 40);
testRef($a);
//The out is :
//Test of reference.

If we remove the "&" from the function,

function testRef($a = array(10, 20, 30))
{
    echo "Test of reference.\n";
}

The same test code:

/*
 *
 *Example 3
 *
 */
testRef(array(10, 30, 40));
//The out is :
//Test of reference.


/*
 *
 *Example 4
 *
 */
$a = array(10, 30, 40);
testRef($a);
//The out is :
//Test of reference.

So we got the note that, when using reference in function, we have first set the value to a variable, then call the function with the variable.