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.

Dynamic Variables in Php

As you know, you create a variable with a dollar sign followed by a variable name.
Unusually, the variable name can itself be stored in a variable. So, when assigning
a value to a variable
$user = "bob";
is equivalent to
$holder="user";
$$holder = "bob";
The $holder variable contains the string "user", so you can think of $$holder as a
dollar sign followed by the value of $holder. PHP interprets this as $user.
Note You can use a string constant to define a dynamic variable instead of a
variable. To do so, you must wrap the string you want to use for the variable
name in braces:
${"user"} = "bob";
This might not seem useful at first glance. However, by using the
concatenation operator and a loop (see Hour 5, "Going with the Flow"), you
can use this technique to create tens of variables dynamically.
When accessing a dynamic variable, the syntax is exactly the same:
$user ="bob";
print $user;
is equivalent to
$user ="bob";
$holder="user";

print $$holder;
If you want to print a dynamic variable within a string, however, you need to give
the interpreter some help. The following print statement:
$user="bob";
$holder="user";
print "$$holder";
does not print "bob" to the browser as you might expect. Instead it prints the strings
"$" and "user" together to make "$user". When you place a variable within
quotation marks, PHP helpfully inserts its value. In this case, PHP replaces $holder
with the string "user". The first dollar sign is left in place. To make it clear to PHP
that a variable within a string is part of a dynamic variable, you must wrap it in
braces. The print statement in the following fragment:
$user="bob";
$holder="user";
print "${$holder}";
now prints "bob", which is the value contained in $user.
Listing 4.1 brings some of the previous code fragments together into a single script
using a string stored in a variable to initialize and access a variable called $user.

Dynamically Setting and Accessing Variables
1: <html>
2: <head>
3: <title>

Dynamically setting and accessing variables</title>
4: </head>
5: <body>
6: <?php
7: $holder = "user";
8: $$holder = "bob";
9:
10: // could have been:
11: // $user = "bob";
12: // ${"user"} = "bob";
13:
14: print "$user<br>"; // prints "bob"
15: print $$holder; // prints "bob"
16: print "<br>";
17: print "${$holder}<br>"; // prints "bob"
18: print "${'user'}<br>"; // prints "bob"
19: ?>

20: </body>
21: </html>