Php_zval_alias is a project mainly written in C, it's free.
Fun with PHP internal value representation, now outdated
Usage:
$ phpize $ ./configure --enable-zval_alias $ make $ php -a -d "extension=./modules/zval_alias.so" Interactive shell
php > $a = 0;
php > $b = 1;
php > var_dump(is_zval_alias('a', 'b'));
bool(false)
php > $b = $a;
php > var_dump(is_zval_alias('a', 'b'));
bool(true)
php > $a = 'rofl';
php > var_dump(is_zval_alias('a', 'b'));
bool(false)
php > $b = &$a;
php > var_dump(is_zval_alias('a', 'b'));
bool(true)
php > $a = 'rifk';
php > var_dump(is_zval_alias('a', 'b'));
bool(true)