Valgrind output:

derick@kossu:~$ valgrind php
==15710== Memcheck, a memory error detector for x86-linux.
==15710== Copyright (C) 2002-2004, and GNU GPL'd, by Julian Seward et al.
==15710== Using valgrind-2.1.2, a program supervision framework for x86-linux.
==15710== Copyright (C) 2000-2004, and GNU GPL'd, by Julian Seward et al.
<?p==15710== For more details, rerun with: -v
==15710==
hp
$my_array = array();
for($x = 0; $x < 2; $x++)
{
      unset($foo);
      $foo = array();
      $foo->bar = "a string";
      $my_array[$x] = $foo;
}
var_dump($my_array);
?>

Warning: Attempt to assign property of non-object in - on line 7

Call Stack:
    0.1031      39896   1. {main}() -:0


Variables in local scope:
  $my_array = array ()
  $x = 0
  $foo = array ()

Warning: Attempt to assign property of non-object in - on line 7

Call Stack:
    0.1031      39896   1. {main}() -:0


Variables in local scope:
  $my_array = array (0 => array ())
  $x = 1
  $foo = array ()
[Mon Oct 11 13:43:59 2004]  Script:  '-'
---------------------------------------
/dat/dev/php/php-5.1dev/Zend/zend_execute.c(80) : Block 0x1C3B81F4 status:
/dat/dev/php/php-5.1dev/Zend/zend_execute.h(64) : Actual location (location was relayed)
Beginning:      Overrun (magic=0x00000007, expected=0x7312F8DC)
      End:      Unknown
---------------------------------------
array(2) {
  [0]=>
  array(0) {
  }
  [1]=>
  array(0) {
  }
}
==15710== Invalid read of size 1
==15710==    at 0x1B904791: strlen (mac_replace_strmem.c:189)
==15710==    by 0x829D50C: xbuf_format_converter (spprintf.c:518)
==15710==    by 0x829DFCA: vspprintf (spprintf.c:725)
==15710==    by 0x1C45C028: xdebug_error_cb (xdebug.c:1527)
==15710==  Address 0x1C3B98B8 is 0 bytes after a block of size 56 alloc'd
==15710==    at 0x1B904EDD: malloc (vg_replace_malloc.c:131)
==15710==    by 0x82C907A: _emalloc (zend_alloc.c:181)
==15710==    by 0x82C9982: _estrndup (zend_alloc.c:418)
==15710==    by 0x82C4E80: lex_scan (zend_language_scanner.l:1511)

Warning: String is not zero-terminated (ZZZZZZZZZZZZZZZZ„̏*) (source: /dat/dev/php/php-5.1dev/Zend/zend_variables.h:35) in Unknown on line 0
[Mon Oct 11 13:44:00 2004]  Script:  '-'
---------------------------------------
/dat/dev/php/php-5.1dev/Zend/zend_variables.h(35) : Block 0x1C3B9880 status:
/dat/dev/php/php-5.1dev/Zend/zend_variables.c(36) : Actual location (location was relayed)
Beginning:      Cached (allocated on /dat/dev/php/php-5.1dev/Zend/zend_vm_handlers.h:2403, 16 bytes)
      End:      OK
---------------------------------------
==15710==
==15710== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 60 from 1)
==15710== malloc/free: in use at exit: 1276 bytes in 10 blocks.
==15710== malloc/free: 11971 allocs, 11961 frees, 909526 bytes allocated.
==15710== For a detailed leak analysis,  rerun with: --leak-check=yes
==15710== For counts of detected errors, rerun with: -v

without xdebug:
derick@kossu:~$ valgrind php
==15759== Memcheck, a memory error detector for x86-linux.
==15759== Copyright (C) 2002-2004, and GNU GPL'd, by Julian Seward et al.
==15759== Using valgrind-2.1.2, a program supervision framework for x86-linux.
==15759== Copyright (C) 2000-2004, and GNU GPL'd, by Julian Seward et al.
==15759== For more details, rerun with: -v
==15759==
<?php
$my_array = array();
for($x = 0; $x < 2; $x++)
{
      unset($foo);
      $foo = array();
      $foo->bar = "a string";
      $my_array[$x] = $foo;
}
var_dump($my_array);
?>

Warning: Attempt to assign property of non-object in - on line 7

Warning: Attempt to assign property of non-object in - on line 7
[Mon Oct 11 13:45:54 2004]  Script:  '-'
---------------------------------------
/dat/dev/php/php-5.1dev/Zend/zend_execute.c(80) : Block 0x1C3B2F7C status:
/dat/dev/php/php-5.1dev/Zend/zend_execute.h(64) : Actual location (location was relayed)
Beginning:      Overrun (magic=0x00000007, expected=0x7312F8DC)
      End:      Unknown
---------------------------------------
array(2) {
  [0]=>
  array(0) {
  }
  [1]=>
  array(0) {
  }
}
==15759== Invalid read of size 1
==15759==    at 0x1B904791: strlen (mac_replace_strmem.c:189)
==15759==    by 0x829D50C: xbuf_format_converter (spprintf.c:518)
==15759==    by 0x829DFCA: vspprintf (spprintf.c:725)
==15759==    by 0x8299914: php_error_cb (main.c:624)
==15759==  Address 0x1C3B4848 is 0 bytes after a block of size 56 alloc'd
==15759==    at 0x1B904EDD: malloc (vg_replace_malloc.c:131)
==15759==    by 0x82C907A: _emalloc (zend_alloc.c:181)
==15759==    by 0x82C9982: _estrndup (zend_alloc.c:418)
==15759==    by 0x82C4E80: lex_scan (zend_language_scanner.l:1511)

Warning: String is not zero-terminated (ZZZZZZZZZZZZZZZZ„̏*) (source: /dat/dev/ php/php-5.1dev/Zend/zend_variables.h:35) in Unknown on line 0
[Mon Oct 11 13:45:54 2004]  Script:  '-'
---------------------------------------
/dat/dev/php/php-5.1dev/Zend/zend_variables.h(35) : Block 0x1C3B4810 status:
/dat/dev/php/php-5.1dev/Zend/zend_variables.c(36) : Actual location (location wa s relayed)
Beginning:      Cached (allocated on /dat/dev/php/php-5.1dev/Zend/zend_vm_handle rs.h:2403, 16 bytes)
      End:      OK
---------------------------------------
==15759==
==15759== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 58 from 1)
==15759== malloc/free: in use at exit: 322 bytes in 4 blocks.
==15759== malloc/free: 11641 allocs, 11637 frees, 882284 bytes allocated.
==15759== For a detailed leak analysis,  rerun with: --leak-check=yes
==15759== For counts of detected errors, rerun with: -v

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.

Should be fixed in 5.0.x CVS. Will apply fix for 5.1.x hopefully tomorrow.