When to pass-by-reference in PHP

Im wondering if its good practice to pass-by-reference when you are only reading a variable, or if it should always be passed as a value.

Example with pass-by-reference:

$a = 'fish and chips';
$b = do_my_hash($a);
echo $b;

function &do_my_hash(&$value){
   return md5($value);
}

Example with pass-by-value:

$a = 'fish and chips';
$b = do_my_hash($a);
echo $b;

function do_my_hash($value){
   return md5($value);
}

Which is better ? E.g if I was to run a loop with 1000 rounds ?

Example of loop:

for($i = 0 ; $i < 1000 ; $i++){
   $a = 'Fish & Chips '.$i;
   echo do_my_hash($a);
}