Which programming languages support variable variables? Can you give some examples?
Variable variables are a feature found in some programming languages that allow the creation and modification of variables dynamically, based on the value of another variable.

Here are some programming languages that support variable variables:

1. PHP - PHP allows the use of variable variables, which are created by prefixing the name of a variable with a dollar sign ($) twice. Here is an example:
```php
$foo = "bar";
$$foo = "hello";
echo $bar; // outputs "hello"
```

2. Perl - In Perl, variable variables are called "symbolic references". They can be created using a scalar variable with the name of the variable to reference, prefixed with a dollar sign ($) and enclosed in curly braces. Here is an example:
```perl
my $foo = "bar";
${$foo} = "hello";
print $bar; # outputs "hello"
```

3. Bash - In Bash, variable variables are created using the eval command, which evaluates a string as a command. Here is an example:
```bash
foo="bar"
bar="hello"
eval echo \$$foo # outputs "hello"
```

It's worth noting that while variable variables can be useful in certain situations, they can also make code harder to read and maintain. As such, it's generally recommended to use them sparingly and only when necessary.