Javascript of http://www.corpuswiki.org translated to PHP and adapted for "restrict universe" option and object orientation.

detect.php

<?php
/**
 * Detect language by trigram sample analysis.
 * Adapted by ppkrauss from http://www.corpuswiki.org/langdetect.html
 */
class LangDetect {
  private $models;
  private $universeConfig;
  private $universeOnDetection;
  private $isoTr3to2;

  public $lang;
  public $lang_alpha2;
  public $score;
  public $score_before;
  public $score_befDiffPerc;
  public $num_langs;

  public function __construct($jsonFile='js/langtrigrams.json', $universe=false) {
    $this->models = json_decode(file_get_contents($jsonFile),true);
    $this->universeConfig = ( is_array($universe) && count($universe) )? $universe: false;
    $this->isoTr3to2=array( // translate ISO 3166-1 codes (from alpha-3 to alpha-2)
		'eng'=>'en', 'por'=>'pt', 'spa'=>'es', 'ita'=>'it', 'fra'=>'fr', 'afr'=>'af',
    );
  }

  public function detect( $content, $universe='' ) { // array('eng','por','spa','ita', 'fra', 'afr')
	if ($universe==='')  $universe = $this->universeConfig;
  	$listScore = $this->scoreLangs($content,$universe); // do compareLang and updates $universeOnDetection
  	$this->score = 0;
	for($i=0; $i<count($listScore); $i++)
		if ( $this->score < $listScore[$i] ) {
			$this->lang = $this->universeOnDetection[$i];
			$this->lang_alpha2 = $this->isoTr3to2[$this->lang];
			$this->score_before = $this->score;
			$this->score = $listScore[$i];
			$this->score_befDiffPerc = round( 100.0*($this->score - $this->score_before)/$this->score );
			// print "\n\t\t-- DEBUG RESULT: bestlang={$this->lang}, score={$this->score} = {$this->models[$i]['iso']}";
		}
	return $this->lang;
  }

  private function scoreLangs($content,$universe='') { 
	$score = array();
	$this->universeOnDetection = array();
	$mindist = $this->num_langs = 0;
  	$contentModel = $this->createOrderedModel($content);
  	foreach ($this->models as $model)  if (!$universe || in_array($model['iso'],$universe)) {
  		$s = $this->compareLang($contentModel, $model['trigrams']);
  		$score[] = $s;
		$this->universeOnDetection[] = $model['iso'];
		$this->num_langs++;
  	}
  	return $score;
  }

  private function createOrderedModel($content) {
	// Create a list of trigrams in content sorted by frequency.
	$trigrams = array();  // assoc
	$sortedTrigrams = array(); // array

	$content = preg_replace('/[^\p{L}\']+/us',' ',$content); // only-UTF8-words filter
	$contentArr = preg_split("//us", strtolower($content), -1, PREG_SPLIT_NO_EMPTY); //str_split_unicode

	for ($i = 0, $l = count($contentArr) - 2; $i < $l; $i++) {
	  $trigramKey = $contentArr[$i] . $contentArr[$i + 1] . $contentArr[$i + 2];
	  if (!isset($trigrams[$trigramKey]))
		$trigrams[$trigramKey] = 1;
	  else
		$trigrams[$trigramKey] += 1;
	}
	// convert object to array:
	foreach ($trigrams as $k=>$v)
	  $sortedTrigrams[count($sortedTrigrams)] = [$k, $v];
	// sort array results:
	uksort($sortedTrigrams, function($objA, $objB) {
		  return $objB[1] - $objA[1]; // sort high-to-low
		}
	);
	return $sortedTrigrams;
  }

  private function compareLang($model, $known_model) {
	// Calculate the distance to the known model.
	$dist = 0;
	if (!is_array($model) || !count($model)) die("\npau com MODEL");
	for ($i = 0, $l = count($model); $i < $l; $i++) {
		if (isset($known_model[$model[$i][0]]) && $known_model[$model[$i][0]]) 
			$dist += abs($model[$i][1] - $known_model[$model[$i][0]]);
		else
			$dist += 300;
	}
	$score = 1 - ( $dist / (300*count($model)));
	return $score;
  }

} // class

Some references and alternatives:
THEORY REF: https://en.wikipedia.org/wiki/Trigram https://en.wikipedia.org/wiki/Language_model#N-gram_models
ALTERNATIVES: https://en.wikipedia.org/wiki/Cache_language_model https://en.wikipedia.org/wiki/Katz%27s_back-off_model