Implementation for a PHP function to find out which variables a given variable is currently referencing or referenced by. This function simply compares the zval pointer by iterating the active symbol table as opposed to simply checking the is_ref property of the zval to tell that it's a reference. This means two-way reference detection is possible.

ref_info.c

/**
*   Find which variables in the current scope reference which other variables.
*   
*   mixed ref_info ( string $variable_name )
*   - Returns an associative array of variable names that are references of the supplied varriable name argument.
*     Returns false on failure.
*/
PHP_FUNCTION(ref_info)
{
    char *str;
    int len;
    long long quickhash = 0;
    zval *sym, **pptr;;
    Bucket *ptrList;
    
    /* The function requires a single argument of type string representative of the symbol name to seek references for */
    if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &str, &len) == FAILURE) {
        return;
    }
    
    if (!len) {
        RETVAL_FALSE;
        return;
    }    
    
    if (!EG(active_symbol_table)) {
        zend_rebuild_symbol_table(TSRMLS_C);
    }
        
    ptrList = EG(active_symbol_table)->pListHead;
    while (ptrList != NULL) {
        if (!strcmp(ptrList->arKey, str)) {
            quickhash = ptrList->pDataPtr;
        }
        ptrList = ptrList->pListNext;
    }
    
    if (!quickhash) {
        RETVAL_FALSE;
        return;
    }

    array_init(return_value);

    ptrList = EG(active_symbol_table)->pListHead;
    while (ptrList != NULL) {
        if (zend_hash_find(EG(active_symbol_table), ptrList->arKey, ptrList->nKeyLength, (void**)&pptr) == SUCCESS) {
            sym = *pptr;
        } else {
            RETVAL_FALSE;
            return;
        }
        if (!sym->is_ref__gc) {
            ptrList = ptrList->pListNext;
            continue;
        } else {
            if (quickhash == ptrList->pDataPtr && strcmp(ptrList->arKey, str)) {
                add_assoc_string(return_value, ptrList->arKey, str, 1);
            }
        }
        ptrList = ptrList->pListNext;
    }
    
}

ref_info.php

<?php

/**
*    Usage example
*/

$a = 42;
$b = &$a;
$c = &$a;
$d = &$c;
$z = "foo bar";

var_dump(ref_info('a'),
         ref_info('b'),
         ref_info('c'),
         ref_info('d'),
         ref_info('e') /* Undefined variable name will return Boolean false */,
         ref_info('z') /* Defined but not a reference will return an empty array */
        );

/**
    Output

array(3) {
  ["b"]=>
  string(1) "a"
  ["c"]=>
  string(1) "a"
  ["d"]=>
  string(1) "a"
}
array(3) {
  ["a"]=>
  string(1) "b"
  ["c"]=>
  string(1) "b"
  ["d"]=>
  string(1) "b"
}
array(3) {
  ["a"]=>
  string(1) "c"
  ["b"]=>
  string(1) "c"
  ["d"]=>
  string(1) "c"
}
array(3) {
  ["a"]=>
  string(1) "d"
  ["b"]=>
  string(1) "d"
  ["c"]=>
  string(1) "d"
}
bool(false)
array(0) {
}

*/