Webmaster in a Nutshell, Second Edition

Variables

In PHP, all variable names begin with a dollar sign ($). The $ is followed by an alphabetic character or an underscore, optionally followed by a sequence of alphanumeric characters and underscores. Variable names in PHP are case-sensitive. Here are some examples:

$i
$counter
$first_name
$_TMP

In PHP, unlike in many other languages, you do not have to explicitly declare variables. PHP automatically declares a variable the first time a value is assigned to it. PHP variables are untyped; you can assign a value of any type to a variable.

Dynamic Variables

Sometimes it is useful to set and use variables dynamically. Normally, you assign a variable like this:

$var = "hello";

Now let’s say you want a variable whose name is the value of the $var variable. You can do that like this:

$$var = "World";

PHP parses $$var by first dereferencing the innermost variable, meaning that $var becomes “hello.” The expression that is left is then $"hello", which is just $hello. In other words, we have just created a new variable named hello and assigned it the value “World.” You can nest dynamic variables to an infinite level in PHP, although once you get beyond two levels, it can be very confusing for someone trying to read your code.

There is a special syntax for using dynamic variables inside quoted strings in PHP:

echo "Hello ${$var}";

This syntax can resolve an ambiguity that occurs when variable arrays are used. Something like $$var[1] is ambiguous because it is impossible for PHP to know which ...