Class to find 'average' image color.

imageColor.php

<?php
/**
 * imageColor 
 * 
 * Shows three methods to find the 'average' image color.
 * 
 * Each function expects a gd image object.
 * 
 * imageColor::averageResize($image) resizing to 1px, and checking the color.
 * imageColor::averageBorder($image) find the average color of all border pixels.
 * imageColor::averageImage($image)  find the average color of all pixels.
 * 
 */
class imageColor
{
	function scanLine($image, $height, $width, $axis, $line)
	{
		$i = 0;
		
		if("x" == $axis){
			$limit = $width;
			$y = $line;
			$x =& $i;
			
			if(-1 == $line){
				$y = 0;
				$y2 = $width -1;
				$x2 =& $i;	
			}
		} else {
			$limit = $height;
			$x = $line;
			$y =& $i;
			
			if(-1 == $line){
				$x = 0;
				$x2 = $width -1;
				$y2 =& $i;	
			}
		}
		
		$colors = array();
		
		if(-1 == $line){
			for($i = 0; $i < $limit; $i++){
				self::addPixel($colors, $image, $x, $y);
				self::addPixel($colors, $image, $x2, $y2);
			}
		} else {
			for($i = 0; $i < $limit; $i++){
				self::addPixel($colors, $image, $x, $y);
			}
		}
		
		return $colors;
	}
	
	function addPixel(&$colors, $image, $x, $y)
	{
		$rgb = imagecolorat($image, $x, $y);
		$color = imagecolorsforindex($image, $rgb);
		$colors['red'][] = $color['red'];
		$colors['green'][] = $color['green'];
		$colors['blue'][] = $color['blue'];
	}
	
	function totalColors($color, $colors)
	{
		$color['red'] += array_sum($colors['red']);
		$color['green'] += array_sum($colors['green']);
		$color['blue'] += array_sum($colors['blue']);

		return $colors;
	}
	
	function averageTotal($color, $count)
	{
		$color['red'] = intval($color['red']/$count);
		$color['green'] = intval($color['green']/$count);
		$color['blue'] = intval($color['blue']/$count);
		
		return $color;
	}
	
	function averageResize($image)
	{
		$width = imagesx($image);
		$height = imagesy($image);
		
		$pixel = imagecreatetruecolor(1, 1);
		imagecopyresampled($pixel, $image, 0, 0, 0, 0, 1, 1, $width, $height);
		$rgb = imagecolorat($pixel, 0, 0);
		$color = imagecolorsforindex($pixel, $rgb);
		
		return $color;
	}
	
	function averageBorder($image)
	{
		$width = imagesx($image);
		$height = imagesy($image);
		
		$colors = self::scanLine($image, $height, $width, 'x', -1);
		self::totalColors(&$color, $colors);
		
		$colors = self::scanLine($image, $height, $width, 'y', -1);
		self::totalColors(&$color, $colors);

		$borderSize = ($height=$width)*2;
		self::averageTotal(&$color, $borderSize);
		
		return $color;
	}
	
	function averageImage($image)
	{
		$width = imagesx($image);
		$height = imagesy($image);
		
		$colors = array();
		
		for($line = 0; $line < $height; $line++){
			$colors = self::scanLine($image, $height, $width, 'x', $line);
			self::totalColors(&$color, $colors);
		}
		
		$count = $width*$height;
		self::averageTotal(&$color, $count);
		
		return $color;
	}
}
?>

The result of the lengthy StackOverflow answer.

Thank you for the nice work, it has been very helpful.
I think you have a little mistake in line 109 "$borderSize = ($height=$width)_2;" should be "$borderSize = ($height+$width)_2;"

;)

Updated for work with PHP 8.1

<?php
/**
 * imageColor 
 * 
 * @link https://gist.github.com/tjlytle/237185 Original source code.
 * 
 * Shows three methods to find the 'average' image color.
 * 
 * Each function expects a gd image object.
 * 
 * imageColor::averageResize($image) resizing to 1px, and checking the color.
 * imageColor::averageBorder($image) find the average color of all border pixels.
 * imageColor::averageImage($image)  find the average color of all pixels.
 * 
 */
class ImageColor
{
	protected static function scanLine($image, $height, $width, $axis, $line)
	{
		$i = 0;
		
		if("x" == $axis){
			$limit = $width;
			$y = $line;
			$x =& $i;
			
			if(-1 == $line){
				$y = 0;
				$y2 = $width -1;
				$x2 =& $i;	
			}
		} else {
			$limit = $height;
			$x = $line;
			$y =& $i;
			
			if(-1 == $line){
				$x = 0;
				$x2 = $width -1;
				$y2 =& $i;	
			}
		}
		
		$colors = array();
		
		if(-1 == $line){
			for($i = 0; $i < $limit; $i++){
				static::addPixel($colors, $image, $x, $y);
				static::addPixel($colors, $image, $x2, $y2);
			}
		} else {
			for($i = 0; $i < $limit; $i++){
				static::addPixel($colors, $image, $x, $y);
			}
		}
		
		return $colors;
	}
	
	protected static function addPixel(&$colors, $image, $x, $y)
	{
		$rgb = imagecolorat($image, $x, $y);
		$color = imagecolorsforindex($image, $rgb);
		$colors['red'][] = $color['red'];
		$colors['green'][] = $color['green'];
		$colors['blue'][] = $color['blue'];
	}
	
	protected static function totalColors(&$color, $colors)
	{
		$color['red'] += array_sum($colors['red']);
		$color['green'] += array_sum($colors['green']);
		$color['blue'] += array_sum($colors['blue']);

		return $colors;
	}
	
	protected static function averageTotal(&$color, $count)
	{
		$color['red'] = intval($color['red']/$count);
		$color['green'] = intval($color['green']/$count);
		$color['blue'] = intval($color['blue']/$count);
		
		return $color;
	}
	
	public static function averageResize($image)
	{
		$width = imagesx($image);
		$height = imagesy($image);
		
		$pixel = imagecreatetruecolor(1, 1);
		imagecopyresampled($pixel, $image, 0, 0, 0, 0, 1, 1, $width, $height);
		$rgb = imagecolorat($pixel, 0, 0);
		$color = imagecolorsforindex($pixel, $rgb);
		
		return $color;
	}
	
	public static function averageBorder($image)
	{
		$width = imagesx($image);
		$height = imagesy($image);

		$pixel = imagecreatetruecolor(1, 1);
		$rgb = imagecolorat($pixel, 0, 0);
		$color = imagecolorsforindex($pixel, $rgb);
		
		$colors = static::scanLine($image, $height, $width, 'x', 0);
		static::totalColors($color, $colors);
		
		$colors = static::scanLine($image, $height, $width, 'y', -1);
		static::totalColors($color, $colors);

		$borderSize = ($height+$width)*2;
		static::averageTotal($color, $borderSize);
		
		return $color;
	}
	
	public static function averageImage($image)
	{
		$width = imagesx($image);
		$height = imagesy($image);
		
		$colors = [];

		$pixel = imagecreatetruecolor(1, 1);
		$rgb = imagecolorat($pixel, 0, 0);
		$color = imagecolorsforindex($pixel, $rgb);
		
		for($line = 0; $line < $height; $line++){
			$colors = static::scanLine($image, $height, $width, 'x', $line);
			static::totalColors($color, $colors);
		}
		
		$count = $width*$height;
		static::averageTotal($color, $count);
		
		return $color;
	}
}

Fixed a lot of errors. I'm not sure is it work correctly compared to original code.