The PHP5 version do not support in Ubuntu 18.04+ versions, so you have to do that configure manually from the source files. If you are using php-5.3.29,
# cd /usr/local/src/php-5.3.29
# ./configure --with-apxs2=/usr/local/apache/bin/apxs --with-mysql=MySQL_LOCATION/mysql --prefix=/usr/local/apache/php --with-config-file-path=/usr/local/apache/php --disable-cgi --with-zlib --with-gettext --with-gdbm --with-curl --enable-zip --with-xml --with-json --enable-shmop
# make
# make install
Restart the Apache server and check phpinfo function on the browser <?php echo phpinfo(); ?>
Note: Please change the MySQL_Location: --with-mysql=MySQL_LOCATION/mysql