Random probability by weight.

PriorityCalculator.php

<?php

class PriorityCalculator
{
    private $data = array();
    private $universe = 0;

    public function add( $data, $probability )
    {
        $this->data[ $x = count( $this->data ) ] = new stdClass;
        $this->data[ $x ]->value = $data;
        $this->universe += $this->data[ $x ]->probability = abs( $probability );
    }

    public function get()
    {
        if( !$this->universe ) 
        {
            return null;
        }
		
        $x = round( mt_rand( 0, $this->universe ) );
        $max = 0;
        $i = 0;

        while( $x > $max )
        {
            $max += $this->data[ $i++ ]->probability;
        }
        
        $val=-1;
        
        if($this->universe==1)
        {
            $val = $this->data[$i]->value;
        } 
        else 
        {
            $val = $this->data[$i-1]->value;
        }
		
        return $val;
    }
}