PHP Linking to a shared library in an extension

I'm working on a native PHP extension which includes symbols from a shared library, but I can't get the config.m4 file to generate a build procedure that includes the necessary linker options to correctly link to the share library. This is what I have in my config.m4 file so far:

PHP_ARG_ENABLE(myextension, whether to enable MyExtension support, [ --enable-myextension   Enable MyExtension support])

if test "$PHP_MYEXTENSION" = "yes"; then

  LIBNAME=otherlibrary
  LIBSYMBOL=otherlib_some_function
  LIBOTHERLIB_LIBS=`pkg-config --libs otherlibrary`
  LIBOTHERLIB_INC=`pkg-config --cflags otherlibrary`

  PHP_CHECK_LIBRARY($LIBNAME,$LIBSYMBOL,
  [
    PHP_EVAL_INCLINE($LIBOTHERLIB_INC)
    PHP_EVAL_LIBLINE($LIBOTHERLIB_LIBS)
    AC_DEFINE(HAVE_MYEXTENSION, 1, [Whether you have MyExtension])
  ],[
    AC_MSG_ERROR([wrong lib$LIBNAME version or library not found])
  ])

  PHP_NEW_EXTENSION(myextension, myextension.c, $ext_shared)
fi

The libotherlibrary library is in a standard system location. When I phpize with this configuration and then execute the generated configure script with the --enable-myextension argument it successfully generates a Makefile etc. and I can see it doing the test for otherlib_some_function from libotherlibrary which passes. But calling ldd on my resulting myextension.so gives just:

linux-vdso.so.1 (0x00007ffd11fce000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f741e653000)
/lib64/ld-linux-x86-64.so.2 (0x00007f741ec26000)

so no mention of libotherlibrary. And the libtool and cc commands which are executed by the Makefile don't include the -lotherlibrary option. And then when I try and execute a PHP script which uses my extension I get:

php: symbol lookup error: /path/to/modules/myext.so: undefined symbol: otherlib_some_function

In place of PHP_EVAL_INCLINE and PHP_EVAL_LIBLINE I've also tried the PHP_ADD_LIBRARY macro:

PHP_ADD_LIBRARY($LIBNAME,,OTHERLIB_SHARED_LIBADD)

(I don't understand what the third argument to this macro is for.)

I've also tried (in combination with both of the above), including the fifth argument to PHP_CHECK_LIBRARY called extra-libs passing in the value of my $LIBOTHERLIB_LIBS variable. Doing so doesn't fix anything, and there's no documentation of what this argument is supposed to do or what values it requires. So this is just a guess really.

So how can I get my extension to build with the required -lotherlibrary linker option?

Solution

My working version now looks like this:

PHP_ARG_ENABLE(myextension, whether to enable MyExtension support, [ --enable-myextension   Enable MyExtension support])

if test "$PHP_MYEXTENSION" = "yes"; then

  LIBNAME=otherlibrary
  LIBSYMBOL=otherlib_some_function
  LIBOTHERLIB_LIBS=`pkg-config --libs otherlibrary`
  LIBOTHERLIB_INC=`pkg-config --cflags otherlibrary`

  PHP_CHECK_LIBRARY($LIBNAME,$LIBSYMBOL,
  [
    PHP_ADD_LIBRARY($LIBNAME,1,OTHERLIBRARY_SHARED_LIBADD)
    PHP_SUBST(OTHERLIBRARY_SHARED_LIBADD)
    AC_DEFINE(HAVE_MYEXTENSION, 1, [Whether you have MyExtension])
  ],[
    AC_MSG_ERROR([wrong lib$LIBNAME version or library not found])
  ])

  PHP_NEW_EXTENSION(myextension, myextension.c, $ext_shared)
fi

This technique uses pkg-config to find the shared library so perhaps it may not be as portable as @hek2mgl's explicit searching.

The main addition is the use of the PHP_SUBST macro which "Adds variable with it's value into Makefile" (according to the aclocal.m4 that phpize created). It seems that the macro PHP_ADD_LIBRARY, documented as "add a library to the link line", doesn't actually add anything to your Makefile. Instead, you supply a variable name as its third argument (documented as "shared-libadd") and later call PHP_SUBST to update that variable in the Makefile.