Weird PHP Yield error

问题

I was trying to get yield working and I copied and pasted the following code from http://php.net/manual/en/language.generators.syntax.php into an empty file and got the error Parse error: syntax error, unexpected '$i' (T_VARIABLE) in [FILENAME]

I'm running XAMPP v3.2.1 which has been working perfectly for the rest of my code (haven't used a yield statement yet) and PHP 5.4.16.

Any idea what I'm doing wrong or what I should do?

<?php
function gen_one_to_three() {
    for ($i = 1; $i <= 3; $i++) {
        // Note that $i is preserved between yields.
        yield $i;
    }
}

$generator = gen_one_to_three();
foreach ($generator as $value) {
    echo "$value\n";
}
?>

the code has no error if you replace yield with echo

回答1:

yield is available only on PHP versions > 5.5.

If you try to use it on previous versions, you'll get a T_VARIABLE parse error.

See 3v4l demo.

回答2:

you must surround the yield statement with parentheses

function gen_one_to_three() {

    for ($i = 1; $i <= 3; $i++) {
        // Note that $i is preserved between yields.
             yield ($i);    
    }

}

$generator = gen_one_to_three();
foreach ($generator as $value) {
    echo "$value\n";
}

来源：https://stackoverflow.com/questions/19332167/weird-php-yield-error