References to Variables in php

By default, variables are assigned by value. In other words, if you were to assign
$aVariable to $anotherVariable, a copy of the value held in $aVariable would be
stored in $anotherVariable. Subsequently changing the value of $aVariable would
have no effect on the contents of $anotherVariable. Listing 4.2 illustrates this.
Listing 4.2: Variables Are Assigned by Value
1: <html>
2: <head>
3: <title>Listing 4.2 Variables are assigned by value</title>
4: </head>
5: <body>
6: <?php
7: $aVariable = 42;
8: $anotherVariable = $aVariable;
9: // a copy of the contents of $aVariable is placed in $anotherVariable
10: $aVariable = 325;
11: print $anotherVariable; // prints 42
12: ?>
13: </body>
14: </html>
This example initializes $aVariable, assigning the value 42 to it. $aVariable is then
assigned to $anotherVariable. A copy of the value of $aVariable is placed in
$anotherVariable. Changing the value of $aVariable to 325 has no effect on the
contents of $anotherVariable. The print statement demonstrates this by outputting
42 to the browser.
In PHP4, you can change this behavior, forcing a reference to $aVariable to be
assigned to $anotherVariable, rather than a copy of its contents. This is illustrated in

1: <html>
2: <head>
3: <title>Listing 4.3 Assigning a variable by reference</title>
4: </head>
5: <body>

6: <?php
7: $aVariable = 42;
8: $anotherVariable = &$aVariable;
9: // a copy of the contents of $aVariable is placed in $anotherVariable
10: $aVariable= 325;
11: print $anotherVariable; // prints 325
12: ?>
13: </body>
14: </html>
We have added only a single character to the code in Listing 4.2. Placing an
ampersand (&) in front of the $aVariable variable ensures that a reference to this
variable, rather than a copy of its contents, is assigned to $anotherVariable. Now
any changes made to $aVariable are seen when accessing $anotherVariable. In
other words, both $aVariable and $anotherVariable now point to the same value.
Because this technique avoids the overhead of copying values from one variable to
another, it can result in a small increase in performance. Unless your script assigns
variables intensively, however, this performance gain will be barely measurable.
Note References to variables were introduced with PHP4.

Adding Comments to PHP Code

Code that seems clear at the time of writing, can seem like a hopeless tangle when
you come to amend it six months later. Adding comments to your code as you write
can save you time later on and make it easier for other programmers to work with
your code.
NEW TERM
A comment is text in a script that is ignored by the interpreter.
Comments can be used to make code more readable, or to annotate a
script.
Single line comments begin with two forward slashes (/ /) or a single hash sign (#).
All text from either of these marks until either the end of the line or the PHP close tag
is ignored.
// this is a comment
# this is another comment
Multiline comments begin with a forward slash followed by an asterisk (/*) and end
with an asterisk followed by a forward slash (*/).
/*
this is a comment
none of this will
be parsed by the
interpreter
*/