Add a 'non-null merge' syntax sugar to PHP7
我们知道从 PHP 5.3 起三元运算符 ? : 有一个写法简洁写法是这样的：
<?php $a = 0; $b = $a ?: 1; # $b === 1
这实际上相当于：
<?php $a = 0; $b = $a ? $a : 1; # $b === 1
在 PHP5 中，语法分析是这样写的：
| expr '?' { zend_do_begin_qm_op(&$1, &$2 TSRMLS_CC); }
expr ':' { zend_do_qm_true(&$4, &$2, &$5 TSRMLS_CC); }
expr { zend_do_qm_false(&$$, &$7, &$2, &$5 TSRMLS_CC); }
| expr '?' ':' { zend_do_jmp_set(&$1, &$2, &$3 TSRMLS_CC); }
expr { zend_do_jmp_set_else(&$$, &$5, &$2, &$3 TSRMLS_CC); }在 PHP7 中，由于 AST（抽象语法树）的引入，语法分析有些简化：
| expr '?' expr ':' expr
{ $$ = zend_ast_create(ZEND_AST_CONDITIONAL, $1, $3, $5); }
| expr '?' ':' expr
{ $$ = zend_ast_create(ZEND_AST_CONDITIONAL, $1, NULL, $4); }PHP7 中语法分析之后都是写到 AST 的节点上。从上面可以看出，简化的 ?: 和完整的三元表达式的区别就是节点中间的值为 NULL。
PHP7 添加了一个合并操作符（T_COALESCE），用于简化 isset 的条件判断：
<?php $b = $a ?? 1;
它相当于：
<?php $b = isset($a) ? $a : 1;
仅仅是 isset 判断，在 为等值时还是会返回b 的值还是为 0 。
这个操作符的语法分析语句是：
| expr T_COALESCE expr
{ $$ = zend_ast_create(ZEND_AST_COALESCE, $1, $3); }如果想将 isset 换成 empty 的效果，也就是说在变量不存在或转换成 boolean 后为 false 都赋予其他值，需要这样写：
<?php $b = $a ?? 1 ?: 1;
显然上面的表达式中中间一部分稍微有些多余，那么做些简化呢？
现在我想添加一个语法 ??: ，它的作用是对变量做 empty 的判断。也就是说达到上面 $a ?? 1 ?: 1 的效果：
<?php $b = $a ??: 1;
改起来很简单，只需要将 ?: 和 ?? 的分析合并一下（注意这里和上面所有的地方 $1 $2 等符号的数字表示的都是变量或者常量出现的位置顺序）：
| expr T_COALESCE ':' expr
{ $$ = zend_ast_create(ZEND_AST_CONDITIONAL,
zend_ast_create(ZEND_AST_COALESCE, $1, $4), NULL, $4); }仅仅只有两句，因为并没有在词法分析器中添加 Token，所以只能算是个语法糖。
重新编译一下之后就能看到效果啦。测试：
$ /usr/local/php/bin/php -r "\$a = 0; echo \$a ?? 1, PHP_EOL;" 0 $ /usr/local/php/bin/php -r "\$a = 0; echo \$a ??: 1, PHP_EOL;" 1
Enjoy IT!
推荐教程：《PHP7》
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