repeated_values.php

$str = "key1=value1&key2=value2&key2=value3&key3=value4";

$params = explode("&", $str);

$output = array();

foreach ($params as $param){
	$param = explode("=", $param);

	$key = $param[0];
	$value = $param[1];

	if(array_key_exists($key, $output))
		$output[$key] = array_merge( (array)$output[$key], (array)$value );
	else
		$output[$key] = $value;
}


print_r($output);