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i have the same problem on different hardware ( p III biproc, Intel(R) Core(TM)2 Duo biproc, AMD Athlon(tm) 64 X2 Dual Core ) 

System is gentoo linux on different kernel.

This is the backtrace on apache execution:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7b90700 (LWP 839)]
0xb6ece496 in load_defaults () from /usr/lib/libmysqlclient.so.15
(gdb) bt
#0  0xb6ece496 in load_defaults () from /usr/lib/libmysqlclient.so.15
#1  0xb73da540 in zm_startup_mysql () from /usr/lib/apache2/modules/libphp5.so
#2  0xb753243e in zend_startup_module_ex () from /usr/lib/apache2/modules/libphp5.so
#3  0xb7539645 in zend_hash_apply () from /usr/lib/apache2/modules/libphp5.so
#4  0xb75325f9 in zend_startup_modules () from /usr/lib/apache2/modules/libphp5.so
#5  0xb74db153 in php_module_startup () from /usr/lib/apache2/modules/libphp5.so
#6  0xb75a8bfc in ?? () from /usr/lib/apache2/modules/libphp5.so
#7  0xb7826d80 in ?? () from /usr/lib/apache2/modules/libphp5.so
#8  0xb7826fe0 in ?? () from /usr/lib/apache2/modules/libphp5.so
#9  0x00000001 in ?? ()
#10 0xb7aea6ba in ?? () from /usr/lib/apache2/modules/mod_ssl.so
#11 0x000000dc in ?? ()
#12 0x0000000b in ?? ()
#13 0xbf9cefd8 in ?? ()
#14 0xb75a8d7e in ?? () from /usr/lib/apache2/modules/libphp5.so
#15 0xb7826d80 in ?? () from /usr/lib/apache2/modules/libphp5.so
#16 0xb77f3f98 in ?? () from /usr/lib/apache2/modules/libphp5.so
#17 0x080a30a0 in ?? ()
#18 0xb7b39ff4 in ?? () from /usr/lib/apache2/modules/mod_headers.so
#19 0x081f6ac8 in ?? ()
#20 0x081f6b38 in ?? ()
#21 0x081f6bf0 in ?? ()
#22 0x000000c8 in ?? ()
#23 0x00000001 in ?? ()
#24 0xb77f3f98 in ?? () from /usr/lib/apache2/modules/libphp5.so
#25 0xbf9cf008 in ?? ()
#26 0x0807a7fe in ap_run_post_config ()
Backtrace stopped: previous frame inner to this frame (corrupt stack?)

Exactly how did you confiugre PHP? What mysql version? (EXACTLY what 
version..)

I have tryed many version of php, mysl and apache as i reported in the 
first post, tring many combination but i have allways the same kind of 
bug.
And this was happen after last system ajournment (gentoo). If i try to 
install others versions backing to older i have ever the same problem.

I repeat: HOW did you configure PHP? What was the configure line you used before you did 'make' ???

ok i have found the problem ... is related on mod_authn_dbd that have a conflict with php+mysql.

To solve it is needeed downgrade apr and apr-util version under 1.3