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深入解析PHP中foreach语句控制数组循环的用法

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Release: 2016-06-06 19:36:30
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这篇文章主要介绍了深入解析PHP中foreach语句控制数组循环的用法,是PHP入门学习中的基础知识,需要的朋友可以参考下

foreach是PHP中很常用的一个用作数组循环的控制语句。
因为它的方便和易用,自然也就在后端隐藏着很复杂的具体实现方式(对用户透明)
今天,我们就来一起分析分析,foreach是如何实现数组(对象)的遍历的。
我们知道PHP是一个脚本语言,也就是说,用户编写的PHP代码最终都是会被PHP解释器解释执行,
特别的,对于PHP来说,所有的用户编写的PHP代码,都会被翻译成PHP的虚拟机ZE的虚拟指令(OPCODES)来执行,不论细节的话,就是说,我们所编写的任何PHP脚本,都会最终被翻译成一条条的指令,从而根据指令,由相应的C编写的函数来执行。

那么foreach会被翻译成什么样子呢?

foreach($arr as $key => $val){ echo $key . '=>' . $val . "\n"; }

在词法分析阶段,foreach会被识别为一个TOKEN:T_FOREACH,
在语法分析阶段,会被规则:

unticked_statement: //没有被绑定ticks的语句 //有省略 | T_FOREACH '(' variable T_AS { zend_do_foreach_begin(&$1, &$2, &$3, &$4, 1 TSRMLS_CC); } foreach_variable foreach_optional_arg ')' { zend_do_foreach_cont(&$1, &$2, &$4, &$6, &$7 TSRMLS_CC); } foreach_statement { zend_do_foreach_end(&$1, &$4 TSRMLS_CC); } | T_FOREACH '(' expr_without_variable T_AS { zend_do_foreach_begin(&$1, &$2, &$3, &$4, 0 TSRMLS_CC); } variable foreach_optional_arg ')' { zend_check_writable_variable(&$6); zend_do_foreach_cont(&$1, &$2, &$4, &$6, &$7 TSRMLS_CC); } foreach_statement { zend_do_foreach_end(&$1, &$4 TSRMLS_CC); } //有省略 ;

仔细分析这段语法规则,我们可以发现,对于:

foreach($arr as $key => $val){ echo $key . ‘=>' . $val .”\n”; }

会被分析为:

T_FOREACH '(' variable T_AS { zend_do_foreach_begin('foreach', '(', $arr, 'as', 1 TSRMLS_CC); } foreach_variable foreach_optional_arg(T_DOUBLE_ARROW foreach_variable) ')' { zend_do_foreach_cont('foreach', '(', 'as', $key, $val TSRMLS_CC); } foreach_satement {zend_do_foreach_end('foreach', 'as');}

然后,让我们来看看foreach_statement:
它其实就是一个代码块,体现了我们的 echo $key . ‘=>' . $val .”\n”;
T_ECHO expr;

显然,实现foreach的核心就是如下3个函数:

zend_do_foreach_begin

zend_do_foreach_cont

zend_do_foreach_end

其中,zend_do_foreach_begin (代码太长,直接写伪码) 主要做了:
1. 记录当前的opline行数(为以后跳转而记录)
2. 对数组进行RESET(讲内部指针指向第一个元素)
3. 获取临时变量 ($val)
4. 设置获取变量的OPCODE FE_FETCH,结果存第3步的临时变量
4. 记录获取变量的OPCODES的行数

而对于 zend_do_foreach_cont来说:
1. 根据foreach_variable的u.EA.type来判断是否引用
2. 根据是否引用来调整zend_do_foreach_begin中生成的FE_FETCH方式
3. 根据zend_do_foreach_begin中记录的取变量的OPCODES的行数,来初始化循环(主要处理在循环内部的循环:do_begin_loop)

最后zend_do_foreach_end:
1. 根据zend_do_foreach_begin中记录的行数信息,设置ZEND_JMP OPCODES
2. 根据当前行数,设置循环体下一条opline, 用以跳出循环
3. 结束循环(处理循环内循环:do_end_loop)
4. 清理临时变量

当然, 在zend_do_foreach_cont 和 zend_do_foreach_end之间 会在语法分析阶段被填充foreach_satement的语句代码。

这样,就实现了foreach的OPCODES line。
比如对于我们开头的实例代码,最终生成的OPCODES是:

filename: /home/huixinchen/foreach.php function name: (null) number of ops: 17 compiled vars: !0 = $arr, !1 = $key, !2 = $val line # op fetch ext return operands ------------------------------------------------------------------------------- 2 0 SEND_VAL 1 1 SEND_VAL 100 2 DO_FCALL 2 'range' 3 ASSIGN !0, $0 3 4 FE_RESET $2 !0, ->14 5 FE_FETCH $3 $2, ->14 6 ZEND_OP_DATA ~5 7 ASSIGN !2, $3 8 ASSIGN !1, ~5 4 9 CONCAT ~7 !1, '-' 10 CONCAT ~8 ~7, !2 11 CONCAT ~9 ~8, '%0A' 12 ECHO ~9 5 13 JMP ->5 14 SWITCH_FREE $2 7 15 RETURN 1 16* ZEND_HANDLE_EXCEPTION

我们注意到FE_FETCH的op2的操作数是14,也就是JMP后一条opline,也就是说,在获取完最后一个数组元素以后,FE_FETCH失败的情况下,会跳到第14行opline,从而实现了循环的结束。
而15行opline的op1的操作数是指向了FE_FETCH,也就是无条件跳转到第5行opline,从而实现了循环。

附录:

void zend_do_foreach_begin(znode *foreach_token, znode *open_brackets_token, znode *array, znode *as_token, int variable TSRMLS_DC) { zend_op *opline; zend_bool is_variable; zend_bool push_container = 0; zend_op dummy_opline; if (variable) { //是否是匿名数组 if (zend_is_function_or_method_call(array)) { //是否是函数返回值 is_variable = 0; } else { is_variable = 1; } /* 使用括号记录FE_RESET的opline行数 */ open_brackets_token->u.opline_num = get_next_op_number(CG(active_op_array)); zend_do_end_variable_parse(BP_VAR_W, 0 TSRMLS_CC); //获取数组/对象和zend_do_begin_variable_parse对应 if (CG(active_op_array)->last > 0 && CG(active_op_array)->opcodes[CG(active_op_array)->last-1].opcode == ZEND_FETCH_OBJ_W) { /* Only lock the container if we are fetching from a real container and not $this */ if (CG(active_op_array)->opcodes[CG(active_op_array)->last-1].op1.op_type == IS_VAR) { CG(active_op_array)->opcodes[CG(active_op_array)->last-1].extended_value |= ZEND_FETCH_ADD_LOCK; push_container = 1; } } } else { is_variable = 0; open_brackets_token->u.opline_num = get_next_op_number(CG(active_op_array)); } foreach_token->u.opline_num = get_next_op_number(CG(active_op_array)); //记录数组Reset Opline number opline = get_next_op(CG(active_op_array) TSRMLS_CC); //生成Reset数组Opcode opline->opcode = ZEND_FE_RESET; opline->result.op_type = IS_VAR; opline->result.u.var = get_temporary_variable(CG(active_op_array)); opline->op1 = *array; SET_UNUSED(opline->op2); opline->extended_value = is_variable ? ZEND_FE_RESET_VARIABLE : 0; dummy_opline.result = opline->result; if (push_container) { dummy_opline.op1 = CG(active_op_array)->opcodes[CG(active_op_array)->last-2].op1; } else { znode tmp; tmp.op_type = IS_UNUSED; dummy_opline.op1 = tmp; } zend_stack_push(&CG(foreach_copy_stack), (void *) &dummy_opline, sizeof(zend_op)); as_token->u.opline_num = get_next_op_number(CG(active_op_array)); //记录循环起始点 opline = get_next_op(CG(active_op_array) TSRMLS_CC); opline->opcode = ZEND_FE_FETCH; opline->result.op_type = IS_VAR; opline->result.u.var = get_temporary_variable(CG(active_op_array)); opline->op1 = dummy_opline.result; //被操作数组 opline->extended_value = 0; SET_UNUSED(opline->op2); opline = get_next_op(CG(active_op_array) TSRMLS_CC); opline->opcode = ZEND_OP_DATA; //当使用key的时候附属操作数,当foreach中不包含key时忽略 SET_UNUSED(opline->op1); SET_UNUSED(opline->op2); SET_UNUSED(opline->result); } void zend_do_foreach_cont(znode *foreach_token, const znode *open_brackets_token, const znode *as_token, znode *value, znode *key TSRMLS_DC) { zend_op *opline; znode dummy, value_node; zend_bool assign_by_ref=0; opline = &CG(active_op_array)->opcodes[as_token->u.opline_num]; //获取FE_FETCH Opline if (key->op_type != IS_UNUSED) { znode *tmp;//交换key和val tmp = key; key = value; value = tmp; opline->extended_value |= ZEND_FE_FETCH_WITH_KEY; //表明需要同时获取key和val } if ((key->op_type != IS_UNUSED) && (key->u.EA.type & ZEND_PARSED_REFERENCE_VARIABLE)) { //key不能以引用方式获取 zend_error(E_COMPILE_ERROR, "Key element cannot be a reference"); } if (value->u.EA.type & ZEND_PARSED_REFERENCE_VARIABLE) { //以引用方式获取值 assign_by_ref = 1; if (!(opline-1)->extended_value) { //根据FE_FETCH的上一条Opline也就是获取数组的扩展值来判断数组是否是匿名数组 zend_error(E_COMPILE_ERROR, "Cannot create references to elements of a temporary array expression"); } opline->extended_value |= ZEND_FE_FETCH_BYREF; //指明按引用取 CG(active_op_array)->opcodes[foreach_token->u.opline_num].extended_value |= ZEND_FE_RESET_REFERENCE; //重置原数组 } else { zend_op *foreach_copy; zend_op *fetch = &CG(active_op_array)->opcodes[foreach_token->u.opline_num]; zend_op *end = &CG(active_op_array)->opcodes[open_brackets_token->u.opline_num]; /* Change "write context" into "read context" */ fetch->extended_value = 0; /* reset ZEND_FE_RESET_VARIABLE */ while (fetch != end) { --fetch; if (fetch->opcode == ZEND_FETCH_DIM_W && fetch->op2.op_type == IS_UNUSED) { zend_error(E_COMPILE_ERROR, "Cannot use [] for reading"); } fetch->opcode -= 3; /* FETCH_W -> FETCH_R */ } /* prevent double SWITCH_FREE */ zend_stack_top(&CG(foreach_copy_stack), (void **) &foreach_copy); foreach_copy->op1.op_type = IS_UNUSED; } value_node = opline->result; if (assign_by_ref) { zend_do_end_variable_parse(value, BP_VAR_W, 0 TSRMLS_CC); //获取值(引用) zend_do_assign_ref(NULL, value, &value_node TSRMLS_CC);//指明value node的type是IS_VAR } else { zend_do_assign(&dummy, value, &value_node TSRMLS_CC); //获取copy值 zend_do_free(&dummy TSRMLS_CC); } if (key->op_type != IS_UNUSED) { znode key_node; opline = &CG(active_op_array)->opcodes[as_token->u.opline_num+1]; opline->result.op_type = IS_TMP_VAR; opline->result.u.EA.type = 0; opline->result.u.opline_num = get_temporary_variable(CG(active_op_array)); key_node = opline->result; zend_do_assign(&dummy, key, &key_node TSRMLS_CC); zend_do_free(&dummy TSRMLS_CC); } do_begin_loop(TSRMLS_C); INC_BPC(CG(active_op_array)); } void zend_do_foreach_end(znode *foreach_token, znode *as_token TSRMLS_DC) { zend_op *container_ptr; zend_op *opline = get_next_op(CG(active_op_array) TSRMLS_CC); //生成JMP opcode opline->opcode = ZEND_JMP; opline->op1.u.opline_num = as_token->u.opline_num; //设置JMP到FE_FETCH opline行 SET_UNUSED(opline->op1); SET_UNUSED(opline->op2); CG(active_op_array)->opcodes[foreach_token->u.opline_num].op2.u.opline_num = get_next_op_number(CG(active_op_array)); //设置跳出循环的opline行 CG(active_op_array)->opcodes[as_token->u.opline_num].op2.u.opline_num = get_next_op_number(CG(active_op_array)); //同上 do_end_loop(as_token->u.opline_num, 1 TSRMLS_CC); //为循环嵌套而设置 zend_stack_top(&CG(foreach_copy_stack), (void **) &container_ptr); generate_free_foreach_copy(container_ptr TSRMLS_CC); zend_stack_del_top(&CG(foreach_copy_stack)); DEC_BPC(CG(active_op_array)); //为PHP interactive模式而设置 }

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