Convert php string to executable code
php字符串转成可执行代码
在php中将字符串转为可执行的代码，可以通过使用php中的“eval()”函数来实现。
eval说明与用法
eval — 把字符串作为PHP代码执行
eval ( string $code ) : mixed
注：函数eval()语言结构是 非常危险的， 因为它允许执行任意 PHP 代码。 它这样用是很危险的。 如果您仔细的确认过，除了使用此结构以外 别无方法, 请多加注意，不要允许传入任何由用户 提供的、未经完整验证过的数据 。
eval参数
code：
需要被执行的字符串
代码不能包含打开/关闭 PHP tags。比如， 'echo "Hi!";' 不能这样传入： ''。但仍然可以用合适的 PHP tag 来离开、重新进入 PHP 模式。比如 'echo "In PHP mode!"; ?>In HTML mode!
除此之外，传入的必须是有效的 PHP 代码。所有的语句必须以分号结尾。比如 'echo "Hi!"' 会导致一个 parse error，而 'echo "Hi!";' 则会正常运行。
return 语句会立即中止当前字符串的执行。
代码执行的作用域是调用 eval() 处的作用域。因此，eval() 里任何的变量定义、修改，都会在函数结束后被保留。
eval返回值
eval() 返回 NULL，除非在执行的代码中 return 了一个值，函数返回传递给 return 的值。 PHP 7 开始，执行的代码里如果有一个 parse error，eval() 会抛出 ParseError 异常。在 PHP 7 之前， 如果在执行的代码中有 parse error，eval() 返回 FALSE，之后的代码将正常执行。无法使用 set_error_handler() 捕获 eval() 中的解析错误。
eval使用示例
<?php
$string = 'cup';
$name = 'coffee';
$str = 'This is a $string with my $name in it.';
echo $str. "\n";
eval("\$str = \"$str\";");
echo $str. "\n";
?>以上例程会输出：
This is a $string with my $name in it.
This is a cup with my coffee in it.
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