Find the frequency of each element in an array in php

Php program for Find the frequency of each element in an array. Here more information.

<?php
class Occurrence
{
	// Function which is display array elements
	public static function display($arr)
	{
		for ($i = 0; $i < count($arr); ++$i)
		{
			printf("%d  ",$arr[$i]);
		}
		printf("\n");
	}
	// Count occurrence of given array
	public static
	function frequency($arr)
	{
		// Display given array
		Occurrence::display($arr);
		// Create a empty map
		$map = array();
		for ($i = 0; $i < count($arr); $i++)
		{
			if (array_key_exists($arr[$i], $map))
			{
				// When key exists then update value
				$map[$arr[$i]] = $map[$arr[$i]] + 1;
			}
			else
			{
				// Add new element
				$map[$arr[$i]] = 1;
			}
		}
		printf("Occurrence \n");
		//  Display calculated result
		foreach(array_keys($map) as $key)
		{
			printf("%d : %d\n",$key,$map[$key]);
		}
	}
	public static
	function main($args)
	{
		// Array element
		$arr = array(1, 3, 2, 1, 4, 2, 7, 9, 1, 3, 3, 4, 7);
		Occurrence::frequency($arr);
	}
}
Occurrence::main(array());

Output

1  3  2  1  4  2  7  9  1  3  3  4  7
Occurrence
1 : 3
3 : 3
2 : 2
4 : 2
7 : 2
9 : 1