Variable variables and how to use illegal variable names
In PHP, the variable $45 is illegal, but if you use variable variables, you can use it. Here is an example,
<?php
$var_name = "45";
$$var_name = "this value will be stored in the 45 variable";
echo ${'45'};
$var_name = "45";
$$var_name = "this value will be stored in the 45 variable";
echo ${'45'};
As you can see, you must use curly bracket syntax to use illegal variables.