PHP中require/include與require_once/include_once的效能對比

_once是要比非once要慢的，因為_once添加了一些條件判斷，在zend的內部hash表中進行尋找，只有當尋找不到的時候才進行包含，
代碼部分見 http://cvs.php.net/viewvc.cgi/ZendEngine2/zend_vm_def.h?view=markup

關鍵抄錄如下：

switch (Z_LVAL(opline->op2.u.constant)) ...{
        case ZEND_INCLUDE_ONCE:
        case ZEND_REQUIRE_ONCE: ...{
                zend_file_handle file_handle;
                char cwd[MAXPATHLEN];
                cwd_state state;

                if (IS_ABSOLUTE_PATH(Z_STRVAL_P(inc_filename), Z_STRLEN_P(inc_filename))) ...{
                    cwd[0] = '';
                } else if (!VCWD_GETCWD(cwd, MAXPATHLEN)) ...{
                    cwd[0] = '';
                }

                state.cwd_length = strlen(cwd);
                state.cwd = zend_strndup(cwd, state.cwd_length);

                failure_retval = (!virtual_file_ex(&state, Z_STRVAL_P(inc_filename), NULL, 1) &&
                    zend_hash_exists(&EG(included_files), state.cwd, state.cwd_length+1));

                free(state.cwd);

                if (failure_retval) ...{
                    /**//* do nothing */
                } else if (SUCCESS == zend_stream_open(Z_STRVAL_P(inc_filename), &file_handle TSRMLS_CC)) ...{
                    if (!file_handle.opened_path) ...{
                        file_handle.opened_path = estrndup(Z_STRVAL_P(inc_filename), Z_STRLEN_P(inc_filename));
                    }

                    if (zend_hash_add_empty_element(&EG(included_files), file_handle.opened_path, strlen(file_handle.opened_path)+1)==SUCCESS) ...{
                        new_op_array = zend_compile_file(&file_handle, (Z_LVAL(opline->op2.u.constant)==ZEND_INCLUDE_ONCE?ZEND_INCLUDE:ZEND_REQUIRE) TSRMLS_CC);
                        zend_destroy_file_handle(&file_handle TSRMLS_CC);
                    } else ...{
                        zend_file_handle_dtor(&file_handle);
                        failure_retval=1;
                    }
                } else ...{
                    if (Z_LVAL(opline->op2.u.constant)==ZEND_INCLUDE_ONCE) ...{
                        zend_message_dispatcher(ZMSG_FAILED_INCLUDE_FOPEN, Z_STRVAL_P(inc_filename));
                    } else ...{
                        zend_message_dispatcher(ZMSG_FAILED_REQUIRE_FOPEN, Z_STRVAL_P(inc_filename));
                    }
                }
            }
            break;
        case ZEND_INCLUDE:
        case ZEND_REQUIRE:
            new_op_array = compile_filename(Z_LVAL(opline->op2.u.constant), inc_filename TSRMLS_CC);
            break;
        case ZEND_EVAL: ...{
                char *eval_desc = zend_make_compiled_string_description("eval()'d code" TSRMLS_CC);

                new_op_array = zend_compile_string(inc_filename, eval_desc TSRMLS_CC);
                efree(eval_desc);
            }
            break;
        EMPTY_SWITCH_DEFAULT_CASE()
    }