i guess the description is descriptive enough.

it happens only with class methods, not with common functions.

example:
<?php

class test {
        function test() { $this->brekeke('a'); }
        function brekeke(&$a) {}
};
$aa = new test();

?>

gdb says:
#0  0x810178d in execute (op_array=0x8330d74) at ./zend_execute.c:1757
1757                                            varptr = *varptr_ptr;
(gdb) bt
#0  0x810178d in execute (op_array=0x8330d74) at ./zend_execute.c:1757
#1  0x810102e in execute (op_array=0x8322b4c) at ./zend_execute.c:1628
#2  0x810102e in execute (op_array=0x82140d4) at ./zend_execute.c:1628
#3  0x810102e in execute (op_array=0x8212ab4) at ./zend_execute.c:1628
#4  0x807ccfc in php_execute_script (primary_file=0xbffffa8c) at main.c:1146
#5  0x80a02a9 in apache_php_module_main (r=0x81f9204, fd=19, 
    display_source_mode=0) at sapi_apache.c:88
#6  0x807835b in send_php ()
#7  0x807839c in send_parsed_php ()
#8  0x8126983 in ap_invoke_handler ()
#9  0x8139eb9 in process_request_internal ()
#10 0x8139f1c in ap_process_request ()
#11 0x81317be in child_main ()
#12 0x81319fc in make_child ()
#13 0x8131d76 in perform_idle_server_maintenance ()
#14 0x81322a5 in standalone_main ()
#15 0x8132863 in main ()
#16 0x4019a94b in __libc_start_main () at ../sysdep