PHP parameter variable vs reference

Question

What is the difference between in php function

• A parameter passage by variable
• A parameter pass by reference?

Answer 1

The best way to understand it is from an example:

function foo($a) {
  $a = 123;
  echo "Value in function: " . $a;
}
function bar(&$a) {
  $a = 123;
  echo "Value in function: " . $a;
}

$var = 555;
foo($var);
echo "After foo: " . $var;
bar($var);
echo "After bar: " . $var;

Basically you will change the value pointed by the reference, changing it also out of the function scope, while in a normal by-value when the function is finished the changes made to the variable will be lost. Here is an official PHP manual link, with more examples.

Answer 2

A parameter passage by value - value of a variable is passed.

$b = 1;
function a($c) {
   $c = 2; // modifying this doesn't change the value of $b, since only the value was passed to $c.
}

a($b);
echo $b; // still outputs 1

A parameter pass by reference? - a pointer to a variable is passed.

$b = 1;
function a($c) {
   $c = 2; // modifying this also changes the value of $b, since variable $c points to it
}

a($b);
echo $b; // outputs 2 now after calling a();