Ubuntu Unleashed 2013 Edition: Covering 12.10 and 13.04, Eighth Edition

References

Using the equal sign (=) copies the value from one variable to another so they both have their own copy of the value. Another option here is to use references, which is where a variable does not have a value of its own; instead, it points to another variable. This enables you to share values and have variables mutually update themselves.

To copy by reference, use the & symbol, as follows:

<?php  $a = 10;  $b = &$a;  echo $a . "\n";  echo $b . "\n";  $a = 20;  echo $a . "\n";  echo $b . "\n";  $b = 30;  echo $a . "\n";  echo $b . "\n";?>

If you run that script, you will see that updating $a also updates $b, but also that updating $b updates $a.