Getting the first character of a string with $str[0]

Question

Yes. Strings can be seen as character arrays, and the way to access a position of an array is to use the [] operator. Usually there's no problem at all in using $str[0] (and I'm pretty sure is much faster than the substr() method).

There is only one caveat with both methods: they will get the first byte, rather than the first character. This is important if you're using multibyte encodings (such as UTF-8). If you want to support that, use mb_substr(). Arguably, you should always assume multibyte input these days, so this is the best option, but it will be slightly slower.

The {} syntax is deprecated as of PHP 5.3.0. Square brackets are recommended.

Lets say you just want the first char from a part of $_POST, lets call it 'type'. And that $_POST['type'] is currently 'Control'. If in this case if you use $_POST['type'][0], or substr($_POST['type'], 0, 1)you will get C back.

However, if the client side were to modify the data they send you, from type to type[] for example, and then send 'Control' and 'Test' as the data for this array, $_POST['type'][0] will now return Control rather than C whereas substr($_POST['type'], 0, 1) will simply just fail.

So yes, there may be a problem with using $str[0], but that depends on the surrounding circumstance.

My only doubt would be how applicable this technique would be on multi-byte strings, but if that's not a consideration, then I suspect you're covered. (If in doubt, mb_substr() seems an obviously safe choice.)

However, from a big picture perspective, I have to wonder how often you need to access the 'n'th character in a string for this to be a key consideration.

It'll vary depending on resources, but you could run the script bellow and see for yourself ;)

<?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;
?>

---

Some reference numbers (on an old CoreDuo machine)

$ 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

It seems that using the [] or {} operators is more or less the same.

$str = 'abcdef';
echo $str[0];                 // a

Speaking as a mere mortal, I would stick with $str[0]. As far as I'm concerned, it's quicker to grasp the meaning of $str[0] at a glance than substr($str, 0, 1). This probably boils down to a matter of preference.

As far as performance goes, well, profile profile profile. :) Or you could peer into the PHP source code...