php-echo原理

1.语法分析

unticked_statement:
|    T_ECHO echo_expr_list ';'
;
echo_expr_list:
        echo_expr_list ',' expr { zend_do_echo(&$ TSRMLS_CC); }
    |    expr                    { zend_do_echo(&$ TSRMLS_CC); }
;
expr:
        r_variable                    { $$ = $; }
    |    expr_without_variable        { $$ = $; }
;
r_variable:
    variable { zend_do_end_variable_parse(&$, BP_VAR_R,  TSRMLS_CC); $$ = $; }
;
variable:
        base_variable_with_function_calls
;
base_variable_with_function_calls:
        base_variable                { $$ = $; }
;
base_variable:
        reference_variable { $$ = $; $$.EA = ZEND_PARSED_VARIABLE; }
;
reference_variable:
    |    compound_variable            { zend_do_begin_variable_parse(TSRMLS_C); fetch_simple_variable(&$$, &$,  TSRMLS_CC); }
;

compound_variable:
        T_VARIABLE            { $$ = $; }
    |    '$' '{' expr '}'    { $$ = $; }
;
    |    compound_variable            { zend_do_begin_variable_parse(TSRMLS_C); fetch_simple_variable(&$$, &$,  TSRMLS_CC); }
;
compound_variable:
        T_VARIABLE            { $$ = $; }

2.编译生成opcode

void zend_do_echo(const znode *arg TSRMLS_DC) /* {{{ */
{
    zend_op *opline = get_next_op(CG(active_op_array) TSRMLS_CC);

    opline->opcode = ZEND_ECHO;
    SET_NODE(opline->op1, arg);
    SET_UNUSED(opline->op2);
}

3.执行已经生成的opcode

static int ZEND_FASTCALL  ZEND_ECHO_SPEC_CV_HANDLER(ZEND_OPCODE_HANDLER_ARGS)
{
    USE_OPLINE

    zval *z;

    SAVE_OPLINE();
    z = _get_zval_ptr_cv_BP_VAR_R(EX_CVs(), opline->op1.var TSRMLS_CC); //这个z已经是值了

    if (IS_CV == IS_TMP_VAR && Z_TYPE_P(z) == IS_OBJECT) {
        INIT_PZVAL(z);
    }
    zend_print_variable(z);

    CHECK_EXCEPTION();
    ZEND_VM_NEXT_OPCODE();
}

static zend_always_inline zval *_get_zval_ptr_cv_BP_VAR_R(zval ***CVs, zend_uint var TSRMLS_DC)
{
    zval ***ptr = &CV(var); //EG(active_op_array).vars[key]该结果是个zend_compile_variable ，其中key为znod_op的变量var,是个数字，可理解为第几个变量，最终在EG(active_sysbole_table)中取出数据，放到EX(CVs)[key]中

    if (UNEXPECTED(*ptr == NULL)) {
        return *_get_zval_cv_lookup_BP_VAR_R(ptr, var TSRMLS_CC);
    }
    return **ptr;
}

static zend_never_inline zval **_get_zval_cv_lookup_BP_VAR_R(zval ***ptr, zend_uint var TSRMLS_DC)
{
    zend_compiled_variable *cv = &CV_DEF_OF(var);

    if (!EG(active_symbol_table) ||
        zend_hash_quick_find(EG(active_symbol_table), cv->name, cv->name_len+, cv->hash_value, (void **)ptr)==FAILURE) {
        zend_error(E_NOTICE, "Undefined variable: %s", cv->name);
        return &EG(uninitialized_zval_ptr);
    }
    return *ptr;
}

ZEND_API int zend_print_variable(zval *var)
{
    return zend_print_zval(var, );
}

ZEND_API int zend_print_zval(zval *expr, int indent) /* {{{ */
{
    return zend_print_zval_ex(zend_write, expr, indent);
}

ZEND_API int zend_print_zval_ex(zend_write_func_t write_func, zval *expr, int indent) /* {{{ */
{
    zval expr_copy;
    int use_copy;

    zend_make_printable_zval(expr, &expr_copy, &use_copy);
    if (use_copy) {
        expr = &expr_copy;
    }
    if (Z_STRLEN_P(expr) == ) { /* optimize away empty strings */
        if (use_copy) {
            zval_dtor(expr);
        }
        return ;
    }
    write_func(Z_STRVAL_P(expr), Z_STRLEN_P(expr));
    if (use_copy) {
        zval_dtor(expr);
    }
    return Z_STRLEN_P(expr);
}

int php_module_startup(sapi_module_struct *sf, zend_module_entry *additional_modules, uint num_additional_modules)
{
    zend_utility_functions zuf;
    zend_utility_values zuv;
    int retval = SUCCESS, module_number=;    /* for REGISTER_INI_ENTRIES() */
    。。。。。。。。。。。。。。

    sapi_module = *sf;

    php_output_startup();

    zuf.error_function = php_error_cb;
    zuf.printf_function = php_printf;
    zuf.write_function = php_output_wrapper;
    zuf.fopen_function = php_fopen_wrapper_for_zend;
    zuf.message_handler = php_message_handler_for_zend;
    zuf.block_interruptions = sapi_module.block_interruptions;
    zuf.unblock_interruptions = sapi_module.unblock_interruptions;
    zuf.get_configuration_directive = php_get_configuration_directive_for_zend;
    zuf.ticks_function = php_run_ticks;
    zuf.on_timeout = php_on_timeout;
    zuf.stream_open_function = php_stream_open_for_zend;
    zuf.vspprintf_function = vspprintf;
    zuf.getenv_function = sapi_getenv;
    zuf.resolve_path_function = php_resolve_path_for_zend;
    zend_startup(&zuf, NULL TSRMLS_CC);

static int php_output_wrapper(const char *str, uint str_length)
{
    TSRMLS_FETCH();
    return php_output_write(str, str_length TSRMLS_CC);
}
PHPAPI int php_output_write(const char *str, size_t len TSRMLS_DC)
{
    if (OG(flags) & PHP_OUTPUT_DISABLED) {
        return ;
    }
    if (OG(flags) & PHP_OUTPUT_ACTIVATED) {
        php_output_op(PHP_OUTPUT_HANDLER_WRITE, str, len TSRMLS_CC);
        return (int) len;
    }
    return php_output_direct(str, len);
}
static int (*php_output_direct)(const char *str, size_t str_len) = php_output_stderr;

static int php_output_stderr(const char *str, size_t str_len)
{
    fwrite(str, 1, str_len, stderr); //可知道echo 是用fwrite输出的
/* See http://support.microsoft.com/kb/190351 */
#ifdef PHP_WIN32
    fflush(stderr);
#endif
    return str_len;
}

参考：http://wenku.baidu.com/view/b7d2d4335a8102d276a22fb1.html