为什么PHP变量在循环周期内是持久的？

The following code should never execute like that in any other language that I know (C, C++, C#, etc.)

<?php

$do = true;

for($i=0; $i<3; $i++) {
    if($do===true) {
        $some_variable = 666;
        echo $some_variable;
        $do = false;
    }
    if(isset($some_variable)) {
        echo $some_variable;
    }
}

Why PHP won't unset a $some_variable in next loop iteration?

Because it is set in the global scope. Once the first iteration sets it, the variable remains set.

http://php.net/manual/en/language.variables.scope.php

Here you can move the work to a function with its own scope. After the first iteration $do is set to false, and it no longer sets the variable:

$do = true;

function do_thing() {
    global $do;
    if($do===true) {
        $some_variable = 666;
        echo $some_variable;
        $do = false;
    }
    if(isset($some_variable)) {
        echo $some_variable;
    }
}

for($i=0; $i<3; $i++) {
    do_thing();
}

Or, you can just make sure you unset the $some_variable in the first iteration.

<?php

$do = true;

for($i=0; $i<3; $i++) {

    if($do===true) {
        $some_variable = 666;
        // echo $some_variable;
        $do = false;
    }
    if(isset($some_variable)) {
        echo $some_variable;
        unset($some_variable);
    }

}

?>

The point is that since you create the $some_variable in the first run of the loop, it will be available in all the subsequent runs of the loop, unless you specifically unset it again.