Programming PHP, 2nd Edition

Variables

Variables in PHP are identifiers prefixed with a dollar sign ($). For example:

    $name
    $Age
    $_debugging
    $MAXIMUM_IMPACT

A variable may hold a value of any type. There is no compile-time or runtime type checking on variables . You can replace a variable’s value with another of a different type:

    $what = "Fred";
    $what = 35;
    $what = array('Fred', '35', 'Wilma');

There is no explicit syntax for declaring variables in PHP. The first time the value of a variable is set, the variable is created. In other words, setting a value to a variable also functions as a declaration. For example, this is a valid complete PHP program:

    $day = 60 * 60 * 24;
    echo "There are $day seconds in a day.\n";
    There are 86400 seconds in a day.

A variable whose value has not been set behaves like the NULL value:

    if ($uninitialized_variable === NULL) {
      echo "Yes!";
    }
    Yes!

Variable Variables

You can reference the value of a variable whose name is stored in another variable. For example:

    $foo = 'bar';
    $$foo = 'baz';

After the second statement executes, the variable $bar has the value "baz".

Variable References

In PHP, references are how you create variable aliases. To make $black an alias for the variable $white, use:

    $black =& $white;

The old value of $black is lost. Instead, $black is now another name for the value that is stored in $white:

 $big_long_variable_name = "PHP"; $short =& $big_long_variable_name; $big_long_variable_name .= " rocks!"; print "\$short is $short\n"; print "Long is $big_long_variable_name\n"; ...