How to install php-xml and php-mbstring on PHP 5.4.9-4?
You should check the package description for PHP from that repo.
Package: libapache2-mod-php5
Source: php5
....
Description: ...
The following extensions are built in: bcmath bz2 calendar Core ctype date
dba dom ereg exif fileinfo filter ftp gettext hash iconv json libxml
mbstring mhash openssl pcre Phar posix Reflection session shmop SimpleXML
soap sockets SPL standard sysvmsg sysvsem sysvshm tokenizer wddx xml
xmlreader xmlwriter zip zlib.
Note that mbstring and xml are built in according to the description.