diff --git a/lib/PerceptualHash/Algorithm/AverageHash.php b/lib/PerceptualHash/Algorithm/AverageHash.php
index f8896af..726b57a 100644
--- a/lib/PerceptualHash/Algorithm/AverageHash.php
+++ b/lib/PerceptualHash/Algorithm/AverageHash.php
@@ -15,15 +15,15 @@ class AverageHash implements Algorithm
     public function bin($resource)
     {
         // Resize
-        $resized = imagecreatetruecolor(static::SIZE, static::SIZE);
+        $resized = imagecreatetruecolor(self::SIZE, self::SIZE);
         imagecopyresampled(
             $resized, $resource, 0, 0, 0, 0,
-            static::SIZE, static::SIZE, imagesx($resource), imagesy($resource));
+            self::SIZE, self::SIZE, imagesx($resource), imagesy($resource));
 
         // Create an array of gray-scaled pixel values.
         $pixels = array();
-        for ($y = 0; $y < static::SIZE; $y++) {
-            for ($x = 0; $x < static::SIZE; $x++) {
+        for ($y = 0; $y < self::SIZE; $y++) {
+            for ($x = 0; $x < self::SIZE; $x++) {
                 $rgb = imagecolorsforindex($resized, imagecolorat($resized, $x, $y));
                 $pixels[] = ($rgb['red'] + $rgb['green'] + $rgb['blue']) / 3;
             }
diff --git a/lib/PerceptualHash/Algorithm/DifferenceHash.php b/lib/PerceptualHash/Algorithm/DifferenceHash.php
index 9418293..30c3532 100644
--- a/lib/PerceptualHash/Algorithm/DifferenceHash.php
+++ b/lib/PerceptualHash/Algorithm/DifferenceHash.php
@@ -14,8 +14,8 @@ class DifferenceHash implements Algorithm
      */
     public function bin($resource)
     {
-        $width = static::SIZE + 1;
-        $heigth = static::SIZE;
+        $width = self::SIZE + 1;
+        $heigth = self::SIZE;
 
         // Resize the image.
         $resized = imagecreatetruecolor($width, $heigth);
diff --git a/lib/PerceptualHash/Algorithm/PerceptionHash.php b/lib/PerceptualHash/Algorithm/PerceptionHash.php
index 755a4ad..a2840cd 100644
--- a/lib/PerceptualHash/Algorithm/PerceptionHash.php
+++ b/lib/PerceptualHash/Algorithm/PerceptionHash.php
@@ -14,16 +14,16 @@ class PerceptionHash implements Algorithm
      */
     public function bin($resource)
     {
-        $resized = imagecreatetruecolor(static::SIZE, static::SIZE);
+        $resized = imagecreatetruecolor(self::SIZE, self::SIZE);
         imagecopyresampled($resized, $resource, 0, 0, 0, 0,
-            static::SIZE, static::SIZE, imagesx($resource), imagesy($resource));
+            self::SIZE, self::SIZE, imagesx($resource), imagesy($resource));
         // Get luma value (YCbCr) from RGB colors and calculate the DCT for each row.
         $matrix = array();
         $row = array();
         $rows = array();
         $col = array();
-        for ($y = 0; $y < static::SIZE; $y++) {
-            for ($x = 0; $x < static::SIZE; $x++) {
+        for ($y = 0; $y < self::SIZE; $y++) {
+            for ($x = 0; $x < self::SIZE; $x++) {
                 $rgb = imagecolorsforindex($resized, imagecolorat($resized, $x, $y));
                 $row[$x] = floor(($rgb['red'] * 0.299) + ($rgb['green'] * 0.587) + ($rgb['blue'] * 0.114));
             }
@@ -32,8 +32,8 @@ class PerceptionHash implements Algorithm
 
         imagedestroy($resized);
 
-        for ($x = 0; $x < static::SIZE; $x++) {
-            for ($y = 0; $y < static::SIZE; $y++) {
+        for ($x = 0; $x < self::SIZE; $x++) {
+            for ($y = 0; $y < self::SIZE; $y++) {
                 $col[$y] = $rows[$y][$x];
             }
             $matrix[$x] = $this->dct($col);