strtolower(); doesn't work for polish chars

<?php strtolower("mĄkA"); ?>
will return: mĄka;

the best solution - use mb_strtolower()

<?php mb_strtolower("mĄkA",'UTF-8'); ?>
will return: mąka

for cyrillic and UTF 8 use  mb_convert_case

exampel

<?php
$string = "Австралия";
$string = mb_convert_case($string, MB_CASE_LOWER, "UTF-8");
echo $string;

//output is: австралия
?>

It is worth noting that 
<?php 
var_dump(strtolower(null))
?>
returns: 
string(0) ""

the function  arraytolower will create duplicate entries since keys are case sensitive.  

<?php
$array = array('test1' => 'asgAFasDAAd', 'TEST2' => 'ASddhshsDGb', 'TeSt3 '=> 'asdasda@asdadadASDASDgh');

$array = arraytolower($array);
?>
/*
Array
(
    [test1] => asgafasdaad
    [TEST2] => ASddhshsDGb
    [TeSt3] => asdasda@asdadadASDASDgh
    [test2] => asddhshsdgb
    [test3] => asdasda@asdadadasdasdgh
)
*/

I prefer this method

<?php
  function arraytolower($array, $include_leys=false) {
    
    if($include_leys) {
      foreach($array as $key => $value) {
        if(is_array($value))
          $array2[strtolower($key)] = arraytolower($value, $include_leys);
        else
          $array2[strtolower($key)] = strtolower($value);
      }
      $array = $array2;
    }
    else {
      foreach($array as $key => $value) {
        if(is_array($value))
          $array[$key] = arraytolower($value, $include_leys);
        else
          $array[$key] = strtolower($value);   
      }
    }
    
    return $array;
  } 
?>

which when used like this

<?php
$array = $array = array('test1' => 'asgAFasDAAd', 'TEST2' => 'ASddhshsDGb', 'TeSt3 '=> 'asdasda@asdadadASDASDgh');

$array1 = arraytolower($array);
$array2 = arraytolower($array,true);

print_r($array1);
print_r($array2);
?>

will give output of 

Array
(
    [test1] => asgafasdaad
    [TEST2] => asddhshsdgb
    [TeSt3] => asdasda@asdadadasdasdgh
)
Array
(
    [test1] => asgafasdaad
    [test2] => asddhshsdgb
    [test3] => asdasda@asdadadasdasdgh
)

When you're not sure, how the current locale is set, you might find the following function useful. It's strtolower for utf8-formatted text:

<?php
function strtolower_utf8($inputString) {
    $outputString    = utf8_decode($inputString);
    $outputString    = strtolower($outputString);
    $outputString    = utf8_encode($outputString);
    return $outputString;
}
?>

It's not suitable for every occasion, but it surely gets in handy. I use it for lowering German 'Umlauts' like ä and ö.

There's a ucfirst "function" to make the first character uppercase, but there's no "lcfirst" function to make the first character lowercase.  Here's my own code to accomplish this.

<?
function lcfirst($str) {
    return strtolower(substr($str, 0, 1)) . substr($str, 1);
}
?>

I found this particularly useful for generating XML nodes with the Reflection class.

<?php
function fullLower($str){
   // convert to entities
   $subject = htmlentities($str,ENT_QUOTES);
   $pattern = '/&([a-z])(uml|acute|circ';
   $pattern.= '|tilde|ring|elig|grave|slash|horn|cedil|th);/e';
   $replace = "'&'.strtolower('\\1').'\\2'.';'";
   $result = preg_replace($pattern, $replace, $subject);
   // convert from entities back to characters
   $htmltable = get_html_translation_table(HTML_ENTITIES);
   foreach($htmltable as $key => $value) {
      $result = ereg_replace(addslashes($value),$key,$result);
   }
   return(strtolower($result));
}

echo fullLower("Ã É Ò Õ ÚÙÛ");

//results ã é ò õ úùû
//adapted from fullUpper on strtoupper manual
?>

Slovenian characters

<?php
    function strtolower_slovenian($string)
    {
        $low=array("Č" => "č", "Ž" => "ž", "Š" => "š");
        return strtolower(strtr($string,$low));
    }

?>

If you're considering using the below unhtmlentities function from phpContrib, I would suggest this one as an alternative:

<?php
function unhtmlentities($string)
{
   // replace numeric entities
   $string = preg_replace('~&#x([0-9a-f]+);~ei', 'chr(hexdec("\\1"))', $string);
   $string = preg_replace('~&#([0-9]+);~e', 'chr(\\1)', $string);
   // replace literal entities
   $trans_tbl = get_html_translation_table(HTML_ENTITIES);
   $trans_tbl = array_flip($trans_tbl);
   return strtr($string, $trans_tbl);
}
?>

That was copied exactly from the html_entity_decode manual page.  It'll handle numeric entities correctly, the below function won't.

Maybe it is not so elegant, but it Works.
It's just a fast Idea and it is what I need.
Any hacks for other characters (link !, ? etc etc) should help.

function RemoveShouting($string)
{
     $frase = "";    
     $astri = explode(".", $string);
     foreach ($astri as $elem)
    $frase .= " ".ucfirst(trim(strtolower($elem))).". ";
       
    return trim($frase);
}

Cheers!
M

Heres a small function I wrote to stop people from submitting data that is ALL IN CAPS SO THEY CAN GET MORE ATTENTION THAT THE REST OF THE USER SUBMITTED DATA on my website :) If you can make it better, by all means do so. This function splits up words delimited by a space, and makes only the first letter of each word capitalized. You can easily modify it so it's only the very first word of the string. I've also added some exceptions so you don't make things like roman numerals look like "Iii" or "Xcmii" or something.

function RemoveShouting($string)
{
 $lower_exceptions = array( 
        "to" => "1", "a" => "1", "the" => "1", "of" => "1"
 ); 
 
 $higher_exceptions = array(  
        "I" => "1", "II" => "1", "III" => "1", "IV" => "1",  
        "V" => "1", "VI" => "1", "VII" => "1", "VIII" => "1",
        "XI" => "1", "X" => "1"
 ); 
 
 $words = split(" ", $string); 
 $newwords = array(); 
 foreach ($words as $word)
 {
  if (!$higher_exceptions[$word]) $word = strtolower($word);
  if (!$lower_exceptions[$word]) $word[0] = strtoupper($word[0]);
  array_push($newwords, $word);
 }
 return join(" ", $newwords);
}

BK

To convert an entire array to lower, I prefer this method;

<?php
function arraytolower(array $array, $round = 0){
  return unserialize(strtolower(serialize($array)));
}
?>

3 lines of code seem a lot less overhead than 10-40.
If there's any intrinsic problem with this method, please post it.

To do case insensitive comparisons in a database, strtolower() can be a quick and dirty solution:

$Sql = "SELECT * FROM tablename WHERE LOWER(column_name) = '".strtolower($my_var)."'";