5.9. Variable Variables
Variable variables are somewhat complicated to use, and even more complicated to explain, so you might need to reread this section a few times before it makes sense! Variable variables allow you to access the contents of a variable without knowing its name directlyit is like indirectly referring to a variable. Here is an example:
$bar = 10;
$foo = "bar"
From that point, there are two ways we can output the value of $bar: we can either use print $bar, which is quite straightforward, or we can take advantage of the concept of variable variables
and use print $$foo;.
By using $$foo, PHP will look up the contents of $foo, convert it to a string, then look up the variable of the same name and return its value. In the example above, $foo contains the string "bar", so PHP will look up the variable named $bar and output its valuein this case, 10. It is possible to use as much indirection as you want, giving variables like $$$foo and $$$$$$$bar. That said, anything beyond one level of indirection can lead to very subtle bugs, and so is best avoided.
Variable variables are often used to choose between two values dynamically, so that the output part of a script references $var but another part of the script actually sets what $var points to. For example, if you have calculated the temperature in Fahrenheit and Celsius and want to choose only one to print out, you might use this code:
$temperature_f = 59;
$temperature_c = 15;
$units = "temperature_f";
$t = $$units;
That assigns the value of $temperature_f to $t.
Variable variables can be helpful from time to time, but are clumsy to use. Furthermore, they only get more clumsy the more indirection you use. For example, the next script outputs "Variable!" four times, but I hope you agree it is not very easy to read:
$foo = "Variable!\n";
$bar = "foo";
$wom = "bar";
$bat = "wom";
print $foo;
print $$bar;
print $$$wom;
print $$$$bat;
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