Is there a way to determine if a variable exists

[Kerry_Kobashi]

Suppose I have an array of symbols, $aSymbols.
After doing an extract($aSymbols), every named variable should be added to PHP symbol table.

$aSymbols["foo"] = "hello";
$aSymbols["foobar"] = "there";
extract($aSymbols);

How can I determine if $foo exists in PHP's symbol table?
Does isset() search PHP's symbol table to check for existence?

[jerdo]

isset($foo)

[remote2]

empty($foo)