How to get a variable name as a string in PHP?

Question

Say i have this PHP code:

$FooBar = \"a string\";

i then need a function like this:

print_var_name($FooBar);

Answer 1

I actually have a valid use case for this.

I have a function cacheVariable($var) (ok, I have a function cache($key, $value), but I'd like to have a function as mentioned).

The purpose is to do:

$colour = 'blue';
cacheVariable($colour);

...

// another session

...

$myColour = getCachedVariable('colour');

I have tried with

function cacheVariable($variable) {
   $key = ${$variable}; // This doesn't help! It only gives 'variable'.
   // do some caching using suitable backend such as apc, memcache or ramdisk
}

I have also tried with

function varName(&$var) {
   $definedVariables = get_defined_vars();
   $copyOfDefinedVariables = array();
   foreach ($definedVariables as $variable=>$value) {
      $copyOfDefinedVariables[$variable] = $value;
   }
   $oldVar = $var;
   $var = !$var;
   $difference = array_diff_assoc($definedVariables, $copyOfDefinedVariables);
   $var = $oldVar;
   return key(array_slice($difference, 0, 1, true));
}

But this fails as well... :(

Sure, I could continue to do cache('colour', $colour), but I'm lazy, you know... ;)

So, what I want is a function that gets the ORIGINAL name of a variable, as it was passed to a function. Inside the function there is no way I'm able to know that, as it seems. Passing get_defined_vars() by reference in the second example above helped me (Thanks to Jean-Jacques Guegan for that idea) somewhat. The latter function started working, but it still only kept returning the local variable ('variable', not 'colour').

I haven't tried yet to use get_func_args() and get_func_arg(), ${}-constructs and key() combined, but I presume it will fail as well.