Apache upgrade problem on Centos 4.1

[cprompt]

Has anyone else had issues running the Apache2 upgrade script on Centos 4.1?
It's always run smoothly on my other boxes running Centos 3.x but this is the first server I've had supplied with Centos 4 and the "build php_ap2" stage fails with the following errors:

ext/standard/html.lo(.text+0x3e): In function `determine_charset':
/usr/local/directadmin/customapache/php-4.3.11/ext/standard/html.c:658: undefined reference to `mbstring_globals'
main/rfc1867.lo(.text+0x350): In function `php_mb_flush_gpc_variables':
/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:58: undefined reference to `php_mb_encoding_translation'
main/rfc1867.lo(.text+0x364):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:59: undefined reference to `php_mb_gpc_encoding_detector'
main/rfc1867.lo(.text+0x3dc):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:61: undefined reference to `php_mb_gpc_encoding_converter'
main/rfc1867.lo(.text+0x761): In function `substring_conf':
/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:591: undefined reference to `php_mb_encoding_translation'
main/rfc1867.lo(.text+0x78c):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:592: undefined reference to `php_mb_gpc_mbchar_bytes'
main/rfc1867.lo(.text+0x7d2): In function `php_ap_getword_conf':
/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:616: undefined reference to `php_mb_encoding_translation'
main/rfc1867.lo(.text+0x839):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:618: undefined reference to `php_mb_gpc_encoding_detector'
main/rfc1867.lo(.text+0xc3a): In function `rfc1867_post_handler':
/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:835: undefined reference to `php_mb_encoding_translation'
main/rfc1867.lo(.text+0xf88):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:900: undefined reference to `php_mb_encoding_translation'
main/rfc1867.lo(.text+0x154c):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:1055: undefined reference to `php_mb_encoding_translation'
main/rfc1867.lo(.text+0x1af2):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:1063: undefined reference to `php_mb_gpc_encoding_detector'
main/rfc1867.lo(.text+0x1b0d):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:1067: undefined reference to `php_mb_strrchr'
main/rfc1867.lo(.text+0x1b1e):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:1068: undefined reference to `php_mb_strrchr'
main/rfc1867.lo(.text+0x1d6a):/usr/local/directadmin/customapache/php-4.3.11/main/rfc1867.c:1065: undefined reference to `php_mb_gpc_encoding_converter'
main/network.lo(.text+0xc10): In function `php_stream_sock_ssl_activate_with_method':
/usr/local/directadmin/customapache/php-4.3.11/main/network.c:755: undefined reference to `php_SSL_new_from_context'
main/network.lo(.text+0xc74):/usr/local/directadmin/customapache/php-4.3.11/main/network.c:789: undefined reference to `php_openssl_apply_verification_policy'
collect2: ld returned 1 exit status
make: *** [sapi/cli/php] Error 1

*** The make has failed, do you want to try to make again? (y,n):

I've Googled around to no avail, so I wondered if anyone else has run into this. It might explain why the hosting company who built the box left it at apache 1.3 !!
If it aint broke don't fix it!