fft.php

<?php
class FourierTransform {
	public $bufferSize;
	public $sampleRate;
	public $bandwidth;
	public $spectrum = array();
	public $real = array();
	public $imag = array();
	public $peakBand = 0;
	public $peak = 0;
	
	public function __construct($bufferSize,$sampleRate){
		$this->bufferSize = $bufferSize;
		$this->sampleRate = $sampleRate;
		$this->bandwidth = 2 / $bufferSize * $sampleRate / 2;
	}
	
	public function getBandFrequency($index){
		return $this->bandwidth * $index + $this->bandwidth / 2;
	}
	
	public function calculateSpectrum(){
		$bSi = 2 / $this->bufferSize;
		for($i = 0,$N = $this->bufferSize/2; $i < $N; $i++){
			$rval = $this->real[$i];
			$ival = $this->imag[$i];
			$mag = $bSi * sqrt($rval * $rval + $ival * $ival);
			if($mag > $this->peak){
				$this->peakBand = $i;
				$this->peak = $mag;
			}
			$this->spectrum[$i] = $mag;
		}
	}
}

class FFT extends FourierTransform {
	public $reverseTable = array();
	public $sinTable = array();
	public $cosTable = array();
	
	public function __construct($bufferSize,$sampleRate){
		parent::__construct($bufferSize,$sampleRate);
		$limit = 1;
		$bit = $bufferSize >> 1;
		while($limit < $bufferSize){
			for($i = 0; $i < $limit; $i++){
				$this->reverseTable[$i + $limit] = $this->reverseTable[$i] + $bit;
			}
			$limit = $limit << 1;
			$bit = $bit >> 1;
		}
		for($i = 0; $i < $bufferSize; $i++){
			$this->sinTable[$i] = sin(-M_PI / $i);
			$this->cosTable[$i] = cos(-M_PI / $i);
		}
	}
	
	public function foward($buffer){
		$k = floor(log($this->bufferSize,2));
		if(pow(2,$k) !== $this->bufferSize) throw new Exception('Invalid buffer size, must be a power of 2.');
		if($this->bufferSize !== count($buffer)) throw new Exception('Supplied buffer is not the same size as defined FFT.');
		
		$halfSize = 1;
		for($i = 0; $i < $this->bufferSize; $i++){
			$this->real[$i] = $buffer[$this->reverseTable[$i]];
			$this->imag[$i] = 0;
		}
		while($halfSize < $this->bufferSize){
			$phaseShiftReal = $this->cosTable[$halfSize];
			$phaseShiftImag = $this->sinTable[$halfSize];
			$currentPhaseShiftReal = 1;
			$currentPhaseShiftImag = 0;
			for($fftStep = 0; $fftStep < $halfSize; $fftStep++){
				while($fftStep < $this->bufferSize){
					$off = $fftStep + $halfSize;
					$tr = ($currentPhaseShiftReal * $this->real[$off]) - ($currentPhaseShiftImag * $this->imag[$off]);
					$ti = ($currentPhaseShiftReal * $this->imag[$off]) + ($currentPhaseShiftImag * $this->real[$off]);
					$this->real[$off] = $this->real[$fftStep] - $tr;
					$this->imag[$off] = $this->imag[$fftStep] - $ti;
					$this->real[$fftStep] += $tr;
					$this->imag[$fftStep] += $ti;
					$fftStep += $halfSize << 1;
				}
				$tmpReal = $currentPhaseShiftReal;
				$currentPhaseShiftReal = ($tmpReal * $phaseShiftReal) - ($currentPhaseShiftImag * $phaseShiftImag);
				$currentPhaseShiftImag = ($tmpReal * $phaseShiftImag) + ($currentPhaseShiftImag * $phaseShiftReal);
			}
			$halfSize = $halfSize << 1;
		}
		$this->calculateSpectrum();
	}
}
?>

spectrum.php

<?php
@set_time_limit(0);
require('fft.php');
$fft = new FFT(1024,44100);
$im = imagecreatetruecolor(200,200);

function genBuffer(){
  $buf = array();
	for($i = 0; $i < 1024; $i++){
		$buf[$i] = ((($i>>7|$i|$i>>6)*10+4*($i&$i>>13|$i>>6))&255);
	}
	return $buf;
}

for($i = 0; $i < 100; $i++){
	$fft->foward(genBuffer());
	foreach($fft->spectrum as $k=>$n){
		imagefilledrectangle($im,$i*imagesx($im)/100,$k*imagesy($im)/count($fft->spectrum),imagesx($im)/100,imagesy($im)/count($fft->spectrum),imagecolorallocate($im,$n*255,$n*255,$n*255));
	}
}

header('Content-type: image/png');
imagepng($im);
?>