PHP Array Syntax Parse Error Left Square Bracket "["

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];
}

. . . with:

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

But, this works:

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

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

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