它会根据资源而有所不同，但你可以运行下面的脚本，自己看看;)

<?php
$tests = 100000;

for ($i = 0; $i < $tests; $i++)
{
    $string = md5(rand());
    $position = rand(0, 31);

    $start1 = microtime(true);
    $char1 = $string[$position];
    $end1 = microtime(true);
    $time1[$i] = $end1 - $start1;

    $start2 = microtime(true);
    $char2 = substr($string, $position, 1);
    $end2 = microtime(true);
    $time2[$i] = $end2 - $start2;

    $start3 = microtime(true);
    $char3 = $string{$position};
    $end3 = microtime(true);
    $time3[$i] = $end3 - $start3;
}

$avg1 = array_sum($time1) / $tests;
echo 'the average float microtime using "array[]" is '. $avg1 . PHP_EOL;

$avg2 = array_sum($time2) / $tests;
echo 'the average float microtime using "substr()" is '. $avg2 . PHP_EOL;

$avg3 = array_sum($time3) / $tests;
echo 'the average float microtime using "array{}" is '. $avg3 . PHP_EOL;
?>

一些参考号(在旧的CoreDuo机器上)

$ php 1.php 
the average float microtime using "array[]" is 1.914701461792E-6
the average float microtime using "substr()" is 2.2536706924438E-6
the average float microtime using "array{}" is 1.821768283844E-6

$ php 1.php 
the average float microtime using "array[]" is 1.7251944541931E-6
the average float microtime using "substr()" is 2.0931363105774E-6
the average float microtime using "array{}" is 1.7225742340088E-6

$ php 1.php 
the average float microtime using "array[]" is 1.7293763160706E-6
the average float microtime using "substr()" is 2.1037721633911E-6
the average float microtime using "array{}" is 1.7249774932861E-6

似乎使用[]或{}操作符或多或少是一样的。

假设你只是想要$_POST的一部分的第一个字符，让我们称之为“类型”。并且$_POST['type']目前是'Control'。如果在这种情况下，如果你使用$_POST['type'][0]，或substr($_POST['type']， 0,1)，你将得到C。

然而，如果客户端要修改他们发送给你的数据，例如从类型到类型[]，然后发送'Control'和'Test'作为该数组的数据，$_POST['type'][0]现在将返回Control而不是C，而substr($_POST['type']， 0,1)将简单地失败。

是的，使用$str[0]可能会有问题，但这取决于周围的环境。

我以前也用过这个符号，没有副作用，也没有误解。这是有道理的——毕竟字符串只是一个字符数组。

它会根据资源而有所不同，但你可以运行下面的脚本，自己看看;)

<?php
$tests = 100000;

for ($i = 0; $i < $tests; $i++)
{
    $string = md5(rand());
    $position = rand(0, 31);

    $start1 = microtime(true);
    $char1 = $string[$position];
    $end1 = microtime(true);
    $time1[$i] = $end1 - $start1;

    $start2 = microtime(true);
    $char2 = substr($string, $position, 1);
    $end2 = microtime(true);
    $time2[$i] = $end2 - $start2;

    $start3 = microtime(true);
    $char3 = $string{$position};
    $end3 = microtime(true);
    $time3[$i] = $end3 - $start3;
}

$avg1 = array_sum($time1) / $tests;
echo 'the average float microtime using "array[]" is '. $avg1 . PHP_EOL;

$avg2 = array_sum($time2) / $tests;
echo 'the average float microtime using "substr()" is '. $avg2 . PHP_EOL;

$avg3 = array_sum($time3) / $tests;
echo 'the average float microtime using "array{}" is '. $avg3 . PHP_EOL;
?>

一些参考号(在旧的CoreDuo机器上)

$ php 1.php 
the average float microtime using "array[]" is 1.914701461792E-6
the average float microtime using "substr()" is 2.2536706924438E-6
the average float microtime using "array{}" is 1.821768283844E-6

$ php 1.php 
the average float microtime using "array[]" is 1.7251944541931E-6
the average float microtime using "substr()" is 2.0931363105774E-6
the average float microtime using "array{}" is 1.7225742340088E-6

$ php 1.php 
the average float microtime using "array[]" is 1.7293763160706E-6
the average float microtime using "substr()" is 2.1037721633911E-6
the average float microtime using "array{}" is 1.7249774932861E-6

似乎使用[]或{}操作符或多或少是一样的。

{}语法从PHP 5.3.0起已弃用。建议使用方括号。

在多字节(unicode)字符串的情况下，使用str[0]可能会导致麻烦。Mb_substr()是一个更好的解决方案。例如:

$first_char = mb_substr($title, 0, 1);

获取UTF-8字符串的第一个字符