load PHP extension module failed

What I would like is to build a *.so , then make it as a PHP extenstion module and call the functions in the *.so via PHP.

My step is as below:

1. Build the C library under linux, first Create the hello.c

   int hello_add(int a, int b)
   {
       return a+b;
   }
   

Then build as below:

    gcc -O -c -fPIC -o hello.o hello.c
    gcc -shared -o libhello.so hello.o

2. download the php 5.2.17 source code

3. tar -zxvf php.5.2.17.tar.gz

4. cd php.5.2.17

5. ./configure ./configure --prefix=/home/user1/php-5.2.17

6. make & make install

7. cd ext;

8. ./ext_skel --extname=hello

9. cd hello

10. edit config.m4 by removing the dnl in line 16-18 then save and quit.

    16: PHP_ARG_ENABLE(hello, whether to enable hello support, 17: dnl Make sure that the comment is aligned: 18: [ --enable-hello Enable hello support])

11. execute the command: /home/user1/php-5.2.17/bin/phpize

12. open php_hello.h, add PHP_FUNCTION(hello_add);

13. open hello.c change to:

    zend_function_entry hello_functions[] = { PHP_FE(confirm_hello_compiled, NULL) /* For testing, remove later. */ PHP_FE(hello_add, NULL) /* For testing, remove later. */ {NULL, NULL, NULL} /* Must be the last line in hello_functions[] */ };

    At the end of the file, add

    PHP_FUNCTION(hello_add) { long int a, b; long int result; if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "ll", &a, &b) == FAILURE) { return; } result = hello_add(a, b); RETURN_LONG(result); }

14. ./configure --with-php-config=/home/usr1/php-5.2.17/bin/php-config

15. make LDFLAGS=-lhello

16. Then the hello.so is generated under /home/usre/php-5.2.17/ext/hello/modules, but use nm hello.so it prints:

                `U hello_add
                 0000000000000a98 T _init`
    

17. Create a php file to test:

    <?php if(!dl('btest.so')) { echo "can not load hello.so"; } else { echo "load is done"; echo hello_add(3,4);// here it will throw error in the log } ?>

in the log, it complains: [28-Sep-2014 18:38:28] PHP Fatal error: Call to undefined function hello_add() ....

BTW, I copied the hello.so into another LAMP environment, not using the PHP just build. Both of the version is 5.2.17.

can anybody point what's going on?