Beim Kompilieren von PHP kam bei mir die Fehlermeldung

checking for libxml-2.0 >= 2.9.0... no
configure: error: Package requirements (libxml-2.0 >= 2.9.0) were not met:

No package 'libxml-2.0' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables LIBXML_CFLAGS
and LIBXML_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.
make: *** No targets specified and no makefile found.  Stop.
make: *** No rule to make target 'install'.  Stop.

Um das Problem zu lösen, musste ich die Umgebungsvariable PKG_CONFIG_PATH wie in der Fehlermeldung beschrieben entsprechend setzen:

export PKG_CONFIG_PATH=/path/to/my/own/libxml2/lib64/pkgconfig:${PKG_CONFIG_PATH}

// ...

./configure --prefix=/path/to/my/own/php
make
make install