To create an extension, you will need at least three files:

• config.m4
• ext_foo.c
• ext_foo.h

Here is a initial content of config.m4:

PHP_ARG_ENABLE(foo, whether to enable foo extension support,
  [--enable-foo Enable foo extension support])
PHP_NEW_EXTENSION(foo, php_foo.c, $ext_shared)

here is the content of php_foo.h:

#ifndef _PHP_FOO_H
#define _PHP_FOO_H
#ifdef HAVE_CONFIG_H
    #include "config.h"
#endif

#ifdef ZTS
    #warning php_ext_uv module will *NEVER* be thread-safe
    #include <TSRM.h>
#endif

#include <php.h>

extern zend_module_entry foo_module_entry;

PHP_FUNCTION(foo_hello);

#endif

Here is the content of php_foo.c, a minimal extension source code must contains a zend_module_entry, And you usually need to define zend_function_entry to declare your extension functions:

#include "php_foo.h"

PHP_MINIT_FUNCTION(foo);
PHP_MSHUTDOWN_FUNCTION(foo);
PHP_MINFO_FUNCTION(foo);

#if COMPILE_DL_FOO
ZEND_GET_MODULE(foo)
#endif

static const zend_function_entry foo_functions[] = {
    PHP_FE(foo_hello, NULL)
    PHP_FE_END
};

zend_module_entry foo_module_entry = {
#if ZEND_MODULE_API_NO >= 20010901
    STANDARD_MODULE_HEADER,
#endif
    "Foo",
    foo_functions, // where you define your functions
    PHP_MINIT(foo),
    PHP_MSHUTDOWN(foo),
    NULL, // PHP_RINIT(foo)
    NULL, // PHP_RSHUTDOWN(foo)
    PHP_MINFO(foo),
#if ZEND_MODULE_API_NO >= 20010901
    "0.1",
#endif
    STANDARD_MODULE_PROPERTIES
};

PHP_MINIT_FUNCTION(foo) {
    return SUCCESS;
}

PHP_MSHUTDOWN_FUNCTION(foo) {
    return SUCCESS;
}

PHP_MINFO_FUNCTION(foo) {
}

PHP_FUNCTION(foo_hello) {
    RETURN_TRUE;
}

Compile it!

phpize
./configure
make