On code one you will have the $black copying any value that the variable $white have. If $white changes it's value later, $black would remain with the same value.
For example:
code 1:
$white = "white";
$black = $white;
echo "\$white = {$white} and \$black = {$black}";
// the result will be $white = white and $black = white
// now changing the value of $white to gray
$white = "gray";
echo "\$white = {$white} and \$black = {$black}";
// the result will be $white = gray and $black = white
Now on code 2 you have the variable by reference, so if the variable changes it's value later, it would affect the variable that receives it as well.
For example:
$white = "white";
$black = &$white;
echo "\$white = {$white} and \$black = {$black}";
// the result will be $white = white and $black = white
// now changing $white value to gray
$white = "gray";
echo "\$white = {$white} and \$black = {$black}";
// the result will be $white = gray and $black = gray
Have fun
