PHP 数组语法解析错误左方括号“[";

我有一个返回数组的函数.我有另一个函数只返回第一行，但由于某种原因，它让我使用一个中间变量，即失败:

I have a function that returns an array. I have another function that just returns the first row, but for some reason, it makes me use an intermediate variable, i.e. this fails:

function f1(/*some args*/) {
    return /*an array*/;
}
function f2(/*some args*/) {
    return f1(/*some args*/)[0];
}

...与:

解析错误:语法错误，第 10 行 util.php 中出现意外的["

Parse error: syntax error, unexpected '[' in util.php on line 10

但是，这是有效的:

function f1(/*some args*/) {
    return /*an array*/;
}
function f2(/*some args*/) {
    $temp = f1(/*some args*/);
    return $temp[0];
}

我无法在网上找到任何相关内容(我的搜索一直被带有?"、{"、<"等的人弄糊涂).

I wasn't able to find anything pertinent online (my searches kept getting confused by people with "?", "{", "<", etc.).

我是自学 PHP - 有什么原因我不能直接做到这一点，但我错过了吗?

I'm self-taught in PHP - is there some reason why I can't do this directly that I've missed?