A small function that allows finding all references to the object. Written in 3 minutes and may be buggy (for ex pass object as reference in some places?)
<?php
function find_ref_obj($object, $obj, $path) {
 if (in_array($obj,$GLOBALS['__REF_CHECKED'],true))
 return false;
 $GLOBALS['__REF_CHECKED'][]=$obj;
 $r = array();
 foreach ((array)$obj as $k => $v) {
 if ($v === $object)
 $r[] = $path . "->$k";
 if (is_object($v)) {
 $t = find_ref_obj($object,$v,$path . "->$k");
 if ($t!==false) 
 $r=array_merge($r,$t);
 }
 else if (is_array($v)) {
 $t = find_ref_arr($object,$v,$path . "->$k");
 if ($t!==false) 
 $r=array_merge($r,$t);
 }
 }
 if (empty($r))
 return false;
 else
 return $r;
}
function find_ref_arr($object, $arr, $path) {
 if (in_array($arr,$GLOBALS['__REF_CHECKED'],true))
 return false; 
 $GLOBALS['__REF_CHECKED'][]=$arr;
 $r = array();
 foreach ($arr as $k => $v) {
 if ($v === $object)
 $r[] = $path . "['$k']";
 if (is_object($v)) {
 $t = find_ref_obj($object,$v,$path . "['$k']");
 if ($t!==false) 
 $r=array_merge($r,$t);
 }
 else if (is_array($v)) {
 $t = find_ref_arr($object,$v,$path . "['$k']");
 if ($t!==false) 
 $r=array_merge($r,$t);
 }
 }
 if (empty($r))
 return false;
 else
 return $r;
}
function find_references($object) {
 $r = array();
 $GLOBALS['__REF_CHECKED']=array();
 foreach ($GLOBALS as $n => $v) 
 if ($n!='__REF_CHECKED')
 if ($n!='GLOBALS') {
 if ($v === $object)
 $r[]=$n;
 if (is_object($v)) {
 $t = find_ref_obj($object,$v,$n);
 if ($t!==false) 
 $r=array_merge($r,$t);
 }
 else if (is_array($v)) {
 $t = find_ref_arr($object,$v,$n);
 if ($t!==false) 
 $r=array_merge($r,$t);
 }
 } 
 unset($GLOBALS['__REF_CHECKED']);
 return $r;
}
function find_refs($object) {
 return implode(', ',find_references($object));
}
?>