Implement PHP_BINARY PHP core predefined constant

Implement PHP_BINARY PHP core predefined constant. PHPUnit uses this constant in its self-test suite, causing many of them to fail on HHVM (possibly only because of the lack of this constant). Try to ninitially implement in such a way that it can be nimble enough for open-source, cross platform.

hphp/util/current_executable.cpp

@@ -0,0 +1,39 @@
+/*
+   +----------------------------------------------------------------------+
+   | HipHop for PHP                                                       |
+   +----------------------------------------------------------------------+
+   | Copyright (c) 2010-2013 Facebook, Inc. (http://www.facebook.com)     |
+   +----------------------------------------------------------------------+
+   | This source file is subject to version 3.01 of the PHP license,      |
+   | that is bundled with this package in the file LICENSE, and is        |
+   | available through the world-wide-web at the following url:           |
+   | http://www.php.net/license/3_01.txt                                  |
+   | If you did not receive a copy of the PHP license and are unable to   |
+   | obtain it through the world-wide-web, please send a note to          |
+   | license@php.net so we can mail you a copy immediately.               |
+   +----------------------------------------------------------------------+
+*/
+
+#include "hphp/util/current_executable.h"
+#include <unistd.h>
+#include <limits.h>
+
+namespace HPHP {
+
+std::string current_executable_path() {
+#ifdef __linux__
+  char result[PATH_MAX];
+  ssize_t count = readlink("/proc/self/exe", result, PATH_MAX);
+  return std::string(result, (count > 0) ? count : 0);
+#elif defined(__APPLE__)
+  char result[PATH_MAX];
+  uint32_t size = sizeof(result);
+  uint32_t success = _NSGetExecutablePath(result, &size);
+  return std:string(result, (success == 0) ? size : 0);
+#else
+  // Return empty string for all other platforms for now (e.g. FreeBSD)
+  return std::string();
+#endif
+}
+
+}