diff --git a/PHP/ShortURL.php b/PHP/ShortURL.php
index 6b03374..d88454b 100644
--- a/PHP/ShortURL.php
+++ b/PHP/ShortURL.php
@@ -26,11 +26,30 @@ class ShortURL {
 	const BASE = 51; // strlen(self::ALPHABET)
 
 	public static function encode($num) {
+		$num = filter_var ( $num, FILTER_VALIDATE_INT, array (
+				'options' => array (
+						'default' => false,
+						'min_range' => 1,
+						'max_range' => PHP_INT_MAX
+				)
+		) );
+		if ($num === false) {
+			throw new InvalidArgumentException ( 'input MUST be an int between 1 and ' . PHP_INT_MAX . ' (PHP_INT_MAX).' );
+		}
 		$str = '';
 
-		while ($num > 0) {
-			$str = self::ALPHABET[($num % self::BASE)] . $str;
-			$num = (int) ($num / self::BASE);
+		while ( $num > 0 ) {
+			$str = self::ALPHABET [($num % self::BASE)] . $str;
+			if($num <= 0x7FFFFFFF){
+				//the number is so small that using regular php division is safe (and fastest)
+				$num = (int) ($num / self::BASE);
+			} else if (is_callable ( 'bcdiv' )) {
+				// use bcmath if available. should be faster than userland php division. could add GMP support.
+				$num = ( int ) bcdiv ( $num, self::BASE, 0 );
+			} else {
+				//fallback to userland php division
+				$num = self::saferudiv ( $num, self::BASE );
+			}
 		}
 
 		return $str;
@@ -38,13 +57,53 @@ public static function encode($num) {
 
 	public static function decode($str) {
 		$num = 0;
-		$len = strlen($str);
+		$len = strlen ( $str );
 
-		for ($i = 0; $i < $len; $i++) {
-			$num = $num * self::BASE + strpos(self::ALPHABET, $str[$i]);
+		for($i = 0; $i < $len; $i ++) {
+			$num = $num * self::BASE + strpos ( self::ALPHABET, $str [$i] );
 		}
 
 		return $num;
 	}
-
+	private static function saferudiv(/*int */$N,/* int */$D) {
+		// based on an algorithm from http://justinparrtech.com/JustinParr-Tech/an-algorithm-for-arbitrary-precision-integer-division/
+		$N = ( int ) abs ( ( int ) $N );
+		$D = ( int ) abs ( ( int ) $D );
+		$M = strlen ( ( string ) $D ) - 1;
+		// assert($N===999999);
+		// assert($D===7777);
+		// assert($M===3);
+		$A = ( int ) ($D - (( int ) ($D % (( int ) pow ( $M, 10 )))));
+		// assert($A===7000);
+		$Q = ( int ) self::_saferudiv ( $N, $A, 0 );
+		// assert($Q===142);
+		$R = ( int ) ($D + 1);
+		// assert($R===7778);
+		while ( (( int ) abs ( $R )) >= $D ) {
+			$R = ( int ) ($N - (( int ) ($Q * $D)));
+			$Qn = ( int ) ($Q + (( int ) (self::_saferudiv ( $R, $A ))));
+			$Q = ( int ) (self::_saferudiv ( ( int ) ($Q + $Qn), 2 ));
+		}
+		$R = ( int ) ($N - (( int ) ($Q * $D)));
+		// assert($R===-3234);
+		if ($R < 0) {
+			$Q = ( int ) ($Q - 1);
+			// assert($Q===128);
+			$R = ( int ) ($R + $D);
+			// assert($R===4543);
+		}
+		// assert(999999===($Q*$D+$R));
+		// var_dump ( $Q, $R );
+		return $Q;
+	}
+	private static function _saferudiv($x, $y, $unused = NULL) {
+		if (PHP_INT_SIZE === 4) {
+			return ( int ) ($x / $y);
+		} elseif (PHP_INT_SIZE === 8) {
+			return ($x - ($x % $y)) / $y;
+		} else {
+			// running 128bit php or something?
+			throw new Exception ( 'this code only supports 4 bytes (32bit) and 8 bytes (64bit) php integers, but this php runtime uses ' . PHP_INT_SIZE . ' bytes integers. the code needs to get updated.' );
+		}
+	}
 }