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    <p>I found a way to adapt <a href="https://github.com/ianbarber/PHPIR/blob/master/holtwinters.php" rel="nofollow">Ian Barber's</a> function to do what I needed.</p> <pre><code>&lt;?php error_reporting(E_ALL); ini_set('display_errors','On'); $anYear1 = array(); $anYear2 = array(); $nStop = 10; for($i = 1; $i &lt;= 12; $i++) { $anYear1[$i] = rand(100,400); if ($i &lt;= $nStop) { $anYear2[$i+12] = rand(200,600); } } print_r($anYear1); print_r($anYear2); $anData = array_merge($anYear1,$anYear2); print_r(forecastHoltWinters($anData)); function forecastHoltWinters($anData, $nForecast = 2, $nSeasonLength = 4, $nAlpha = 0.2, $nBeta = 0.01, $nGamma = 0.01, $nDevGamma = 0.1) { // Calculate an initial trend level $nTrend1 = 0; for($i = 0; $i &lt; $nSeasonLength; $i++) { $nTrend1 += $anData[$i]; } $nTrend1 /= $nSeasonLength; $nTrend2 = 0; for($i = $nSeasonLength; $i &lt; 2*$nSeasonLength; $i++) { $nTrend2 += $anData[$i]; } $nTrend2 /= $nSeasonLength; $nInitialTrend = ($nTrend2 - $nTrend1) / $nSeasonLength; // Take the first value as the initial level $nInitialLevel = $anData[0]; // Build index $anIndex = array(); foreach($anData as $nKey =&gt; $nVal) { $anIndex[$nKey] = $nVal / ($nInitialLevel + ($nKey + 1) * $nInitialTrend); } // Build season buffer $anSeason = array_fill(0, count($anData), 0); for($i = 0; $i &lt; $nSeasonLength; $i++) { $anSeason[$i] = ($anIndex[$i] + $anIndex[$i+$nSeasonLength]) / 2; } // Normalise season $nSeasonFactor = $nSeasonLength / array_sum($anSeason); foreach($anSeason as $nKey =&gt; $nVal) { $anSeason[$nKey] *= $nSeasonFactor; } $anHoltWinters = array(); $anDeviations = array(); $nAlphaLevel = $nInitialLevel; $nBetaTrend = $nInitialTrend; foreach($anData as $nKey =&gt; $nVal) { $nTempLevel = $nAlphaLevel; $nTempTrend = $nBetaTrend; $nAlphaLevel = $nAlpha * $nVal / $anSeason[$nKey] + (1.0 - $nAlpha) * ($nTempLevel + $nTempTrend); $nBetaTrend = $nBeta * ($nAlphaLevel - $nTempLevel) + ( 1.0 - $nBeta ) * $nTempTrend; $anSeason[$nKey + $nSeasonLength] = $nGamma * $nVal / $nAlphaLevel + (1.0 - $nGamma) * $anSeason[$nKey]; $anHoltWinters[$nKey] = ($nAlphaLevel + $nBetaTrend * ($nKey + 1)) * $anSeason[$nKey]; $anDeviations[$nKey] = $nDevGamma * abs($nVal - $anHoltWinters[$nKey]) + (1-$nDevGamma) * (isset($anDeviations[$nKey - $nSeasonLength]) ? $anDeviations[$nKey - $nSeasonLength] : 0); } $anForecast = array(); $nLast = end($anData); for($i = 1; $i &lt;= $nForecast; $i++) { $nComputed = round($nAlphaLevel + $nBetaTrend * $anSeason[$nKey + $i]); if ($nComputed &lt; 0) { // wildly off due to outliers $nComputed = $nLast; } $anForecast[] = $nComputed; } return $anForecast; } </code></pre>
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    1. POHow do I use Holt-Winters Seasonal Dampened Method to compute a two-month sales projection in PHP?
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    1. POHow do I use Holt-Winters Seasonal Dampened Method to compute a two-month sales projection in PHP?
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