<?php
$filter = "select_time=1&grade1=1&grade2=2&grade3=3&genre5=5&genre19=19&language0=0";
$array1 = explode("&", $filter);
$array2 = array();

foreach ($array1 as $cadena){
	$clave = substr($cadena, 0, strpos($cadena, "=") - 1);
	$valor = substr($cadena, strpos($cadena, "=") + 1);
	$array2[$clave] = $valor;
}

print_r($array2);