What is the best way to generate a random key within PHP?

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Update (12/2015): For PHP 7.0, you should use random_int() instead of mt_rand as it provides “cryptographically secure values”

Personally, I like to use sha1(microtime(true).mt_rand(10000,90000)) but you are looking for more of a customizable approach, so try this function (which is a modification to your request of this answer):

function rand_char($length) {
  $random = '';
  for ($i = 0; $i < $length; $i++) {
    $random .= chr(mt_rand(33, 126));
  }
  return $random;
}

Still, this will probably be significantly slower than uniqid(), md5(), or sha1().

Edit: Looks like you got to it first, sorry. 😀

Edit 2: I decided to do a nice little test on my Debian machine with PHP 5 and eAccelerator (excuse the long code):

function rand_char($length) {
  $random = '';
  for ($i = 0; $i < $length; $i++) {
    $random .= chr(mt_rand(33, 126));
  }
  return $random;
}

function rand_sha1($length) {
  $max = ceil($length / 40);
  $random = '';
  for ($i = 0; $i < $max; $i ++) {
    $random .= sha1(microtime(true).mt_rand(10000,90000));
  }
  return substr($random, 0, $length);
}

function rand_md5($length) {
  $max = ceil($length / 32);
  $random = '';
  for ($i = 0; $i < $max; $i ++) {
    $random .= md5(microtime(true).mt_rand(10000,90000));
  }
  return substr($random, 0, $length);
}

$a = microtime(true);
for ($x = 0; $x < 1000; $x++)
  $temp = rand_char(1000);

echo "Rand:\t".(microtime(true) - $a)."\n";

$a = microtime(true);
for ($x = 0; $x < 1000; $x++)
  $temp = rand_sha1(1000);

echo "SHA-1:\t".(microtime(true) - $a)."\n";

$a = microtime(true);
for ($x = 0; $x < 1000; $x++)
  $temp = rand_md5(1000);

echo "MD5:\t".(microtime(true) - $a)."\n";

Results:

Rand:   2.09621596336
SHA-1:  0.611464977264
MD5:    0.618473052979

So my suggestion, if you want speed (but not full charset), is to stick to MD5, SHA-1, or Uniqid (which I didn’t test.. yet)

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