If you try to use mb_detect_encoding to detect whether a string is valid UTF-8, use the strict mode, it is pretty worthless otherwise.
<?php
 $str = 'áéóú'; // ISO-8859-1
 mb_detect_encoding($str, 'UTF-8'); // 'UTF-8'
 mb_detect_encoding($str, 'UTF-8', true); // false
?>

The documentation is no longer correct for php8.1 and mb_detect_encoding no longer supports order of encodings. The example outputs given in the documentation are also no longer correct for php8.1. This is somewhat explained here https://github.com/php/php-src/issues/8279
I understand the previous ambiguity in these functions, but in my option 8.1 should have deprecated mb_detect_encoding and mb_detect_order and came up with different functions. It now tries to find the encoding that will use the least amount of space regardless of the order, and I am not sure who needs that.
Below is an example function that will do what mb_detect_encoding was doing prior to the 8.1 change.
<?php
function mb_detect_enconding_in_order(string $string, array $encodings): string|false
{
 foreach($encodings as $enc) {
 if (mb_check_encoding($string, $enc)) {
 return $enc;
 }
 }
 return false;
}
?>

Major undocumented breaking change since 8.1.7
https://3v4l.org/BLjZ3
Make sure to replace mb_detect_encoding with a loop of calls to mb_check_encoding

If you need to distinguish between UTF-8 and ISO-8859-1 encoding, list UTF-8 first in your encoding_list:
mb_detect_encoding($string, 'UTF-8, ISO-8859-1');
if you list ISO-8859-1 first, mb_detect_encoding() will always return ISO-8859-1.

Based upon that snippet below using preg_match() I needed something faster and less specific. That function works and is brilliant but it scans the entire strings and checks that it conforms to UTF-8. I wanted something purely to check if a string contains UTF-8 characters so that I could switch character encoding from iso-8859-1 to utf-8.
I modified the pattern to only look for non-ascii multibyte sequences in the UTF-8 range and also to stop once it finds at least one multibytes string. This is quite a lot faster.
<?php
function detectUTF8($string)
{
 return preg_match('%(?:
 [\xC2-\xDF][\x80-\xBF] # non-overlong 2-byte
 |\xE0[\xA0-\xBF][\x80-\xBF] # excluding overlongs
 |[\xE1-\xEC\xEE\xEF][\x80-\xBF]{2} # straight 3-byte
 |\xED[\x80-\x9F][\x80-\xBF] # excluding surrogates
 |\xF0[\x90-\xBF][\x80-\xBF]{2} # planes 1-3
 |[\xF1-\xF3][\x80-\xBF]{3} # planes 4-15
 |\xF4[\x80-\x8F][\x80-\xBF]{2} # plane 16
 )+%xs', $string);
}
?>

A simple way to detect UTF-8/16/32 of file by its BOM (not work with string or file without BOM)
<?php
// Unicode BOM is U+FEFF, but after encoded, it will look like this.
define ('UTF32_BIG_ENDIAN_BOM' , chr(0x00) . chr(0x00) . chr(0xFE) . chr(0xFF));
define ('UTF32_LITTLE_ENDIAN_BOM', chr(0xFF) . chr(0xFE) . chr(0x00) . chr(0x00));
define ('UTF16_BIG_ENDIAN_BOM' , chr(0xFE) . chr(0xFF));
define ('UTF16_LITTLE_ENDIAN_BOM', chr(0xFF) . chr(0xFE));
define ('UTF8_BOM' , chr(0xEF) . chr(0xBB) . chr(0xBF));
function detect_utf_encoding($filename) {
 $text = file_get_contents($filename);
 $first2 = substr($text, 0, 2);
 $first3 = substr($text, 0, 3);
 $first4 = substr($text, 0, 3);
 
 if ($first3 == UTF8_BOM) return 'UTF-8';
 elseif ($first4 == UTF32_BIG_ENDIAN_BOM) return 'UTF-32BE';
 elseif ($first4 == UTF32_LITTLE_ENDIAN_BOM) return 'UTF-32LE';
 elseif ($first2 == UTF16_BIG_ENDIAN_BOM) return 'UTF-16BE';
 elseif ($first2 == UTF16_LITTLE_ENDIAN_BOM) return 'UTF-16LE';
}
?>

Beware of bug to detect Russian encodings
http://bugs.php.net/bug.php?id=38138 

I used Chris's function "detectUTF8" to detect the need from conversion from utf8 to 8859-1, which works fine. I did have a problem with the following iconv-conversion.
The problem is that the iconv-conversion to 8859-1 (with //TRANSLIT) replaces the euro-sign with EUR, although it is common practice that \x80 is used as the euro-sign in the 8859-1 charset. 
I could not use 8859-15 since that mangled some other characters, so I added 2 str_replace's:
if(detectUTF8($str)){
 $str=str_replace("\xE2\x82\xAC","€",$str); 
 $str=iconv("UTF-8","ISO-8859-1//TRANSLIT",$str);
 $str=str_replace("€","\x80",$str); 
}
If html-output is needed the last line is not necessary (and even unwanted).

Just a note: Instead of using the often recommended (rather complex) regular expression by W3C (http://www.w3.org/International/questions/qa-forms-utf-8.en.php), you can simply use the 'u' modifier to test a string for UTF-8 validity:
<?php
 if (preg_match("//u", $string)) {
 // $string is valid UTF-8
 }

Function to detect UTF-8, when mb_detect_encoding is not available it may be useful.
<?php
function is_utf8($str) {
 $c=0; $b=0;
 $bits=0;
 $len=strlen($str);
 for($i=0; $i<$len; $i++){
 $c=ord($str[$i]);
 if($c > 128){
 if(($c >= 254)) return false;
 elseif($c >= 252) $bits=6;
 elseif($c >= 248) $bits=5;
 elseif($c >= 240) $bits=4;
 elseif($c >= 224) $bits=3;
 elseif($c >= 192) $bits=2;
 else return false;
 if(($i+$bits) > $len) return false;
 while($bits > 1){
 $i++;
 $b=ord($str[$i]);
 if($b < 128 || $b > 191) return false;
 $bits--;
 }
 }
 }
 return true;
}
?>

Much simpler UTF-8-ness checker using a regular expression created by the W3C:
<?php
// Returns true if $string is valid UTF-8 and false otherwise.
function is_utf8($string) {
 
 // From http://w3.org/International/questions/qa-forms-utf-8.html
 return preg_match('%^(?:
 [\x09\x0A\x0D\x20-\x7E] # ASCII
 | [\xC2-\xDF][\x80-\xBF] # non-overlong 2-byte
 | \xE0[\xA0-\xBF][\x80-\xBF] # excluding overlongs
 | [\xE1-\xEC\xEE\xEF][\x80-\xBF]{2} # straight 3-byte
 | \xED[\x80-\x9F][\x80-\xBF] # excluding surrogates
 | \xF0[\x90-\xBF][\x80-\xBF]{2} # planes 1-3
 | [\xF1-\xF3][\x80-\xBF]{3} # planes 4-15
 | \xF4[\x80-\x8F][\x80-\xBF]{2} # plane 16
 )*$%xs', $string);
 
} // function is_utf8
?>

For detect UTF-8, you can use:
if (preg_match('!!u', $str)) { echo 'utf-8'; }
- Norihiori

It was helpful for my exec(...) call. When it returned cp866 or cp1251:
try {
 $line = iconv('CP866', 'CP1251', $line);
} catch(Exception $e) {
}
return iconv('CP1251', 'UTF-8', $line);

if the function " mb_detect_encoding" does not exist ... 
... try: 
<?php 
// ---------------------------------------------------- 
if ( !function_exists('mb_detect_encoding') ) { 
// ---------------------------------------------------------------- 
function mb_detect_encoding ($string, $enc=null, $ret=null) { 
 
 static $enclist = array( 
 'UTF-8', 'ASCII', 
 'ISO-8859-1', 'ISO-8859-2', 'ISO-8859-3', 'ISO-8859-4', 'ISO-8859-5', 
 'ISO-8859-6', 'ISO-8859-7', 'ISO-8859-8', 'ISO-8859-9', 'ISO-8859-10', 
 'ISO-8859-13', 'ISO-8859-14', 'ISO-8859-15', 'ISO-8859-16', 
 'Windows-1251', 'Windows-1252', 'Windows-1254', 
 );
 
 $result = false; 
 
 foreach ($enclist as $item) { 
 $sample = iconv($item, $item, $string); 
 if (md5($sample) == md5($string)) { 
 if ($ret === NULL) { $result = $item; } else { $result = true; } 
 break; 
 }
 }
 
 return $result; 
} 
// ---------------------------------------------------------------- 
} 
// ---------------------------------------------------- 
?>

example / usage of: mb_detect_encoding() 
<?php 
// ------------------------------------------------------ 
function str_to_utf8 ($str) { 
 
 if (mb_detect_encoding($str, 'UTF-8', true) === false) { 
 $str = utf8_encode($str); 
 }
 return $str;
}
// ------------------------------------------------------ 
?>

$txtstr = str_to_utf8($txtstr);

Sometimes mb_detect_string is not what you need. When using pdflib for example you want to VERIFY the correctness of utf-8. mb_detect_encoding reports some iso-8859-1 encoded text as utf-8.
To verify utf 8 use the following:
//
// utf8 encoding validation developed based on Wikipedia entry at:
// http://en.wikipedia.org/wiki/UTF-8
//
// Implemented as a recursive descent parser based on a simple state machine
// copyright 2005 Maarten Meijer
//
// This cries out for a C-implementation to be included in PHP core
//
 function valid_1byte($char) {
 if(!is_int($char)) return false;
 return ($char & 0x80) == 0x00;
 }
 
 function valid_2byte($char) {
 if(!is_int($char)) return false;
 return ($char & 0xE0) == 0xC0;
 }
 function valid_3byte($char) {
 if(!is_int($char)) return false;
 return ($char & 0xF0) == 0xE0;
 }
 function valid_4byte($char) {
 if(!is_int($char)) return false;
 return ($char & 0xF8) == 0xF0;
 }
 
 function valid_nextbyte($char) {
 if(!is_int($char)) return false;
 return ($char & 0xC0) == 0x80;
 }
 
 function valid_utf8($string) {
 $len = strlen($string);
 $i = 0; 
 while( $i < $len ) {
 $char = ord(substr($string, $i++, 1));
 if(valid_1byte($char)) { // continue
 continue;
 } else if(valid_2byte($char)) { // check 1 byte
 if(!valid_nextbyte(ord(substr($string, $i++, 1))))
 return false;
 } else if(valid_3byte($char)) { // check 2 bytes
 if(!valid_nextbyte(ord(substr($string, $i++, 1))))
 return false;
 if(!valid_nextbyte(ord(substr($string, $i++, 1))))
 return false;
 } else if(valid_4byte($char)) { // check 3 bytes
 if(!valid_nextbyte(ord(substr($string, $i++, 1))))
 return false;
 if(!valid_nextbyte(ord(substr($string, $i++, 1))))
 return false;
 if(!valid_nextbyte(ord(substr($string, $i++, 1))))
 return false;
 } // goto next char
 }
 return true; // done
 }
for a drawing of the statemachine see: http://www.xs4all.nl/~mjmeijer/unicode.png and http://www.xs4all.nl/~mjmeijer/unicode2.png 

a) if the FUNCTION mb_detect_encoding is not available: 
### mb_detect_encoding ... iconv ###
<?php
// -------------------------------------------
if(!function_exists('mb_detect_encoding')) { 
function mb_detect_encoding($string, $enc=null) { 
 
 static $list = array('utf-8', 'iso-8859-1', 'windows-1251');
 
 foreach ($list as $item) {
 $sample = iconv($item, $item, $string);
 if (md5($sample) == md5($string)) { 
 if ($enc == $item) { return true; } else { return $item; } 
 }
 }
 return null;
}
}
// -------------------------------------------
?>

b) if the FUNCTION mb_convert_encoding is not available: 
### mb_convert_encoding ... iconv ###
<?php
// -------------------------------------------
if(!function_exists('mb_convert_encoding')) { 
function mb_convert_encoding($string, $target_encoding, $source_encoding) { 
 $string = iconv($source_encoding, $target_encoding, $string); 
 return $string; 
}
}
// -------------------------------------------
?>

beware : even if you need to distinguish between UTF-8 and ISO-8859-1, and you the following detection order (as chrigu suggests)
mb_detect_encoding('accentu?e' , 'UTF-8, ISO-8859-1')
returns ISO-8859-1, while 
mb_detect_encoding('accentu?' , 'UTF-8, ISO-8859-1')
returns UTF-8
bottom line : an ending '?' (and probably other accentuated chars) mislead mb_detect_encoding

In my environment (PHP 7.1.12),
"mb_detect_encoding()" doesn't work
 where "mb_detect_order()" is not set appropriately.
To enable "mb_detect_encoding()" to work in such a case,
 simply put "mb_detect_order('...')"
 before "mb_detect_encoding()" in your script file.
Both 
 "ini_set('mbstring.language', '...');"
 and
 "ini_set('mbstring.detect_order', '...');"
DON'T work in script files for this purpose
whereas setting them in PHP.INI file may work.

About function mb_detect_encoding, the link http://php.net/manual/zh/function.mb-detect-encoding.php , like this:
mb_detect_encoding('áéóú', 'UTF-8', true); // false
but now the result is not false, can you give me reason, thanks!