<?php

    $data1 = '20-09-16';
    $data2 = '23-09-16';
    
    $data1 = explode( '-', $data1 );
    $data2 = explode( '-', $data2 );

    echo $dia1 = mktime( 0, 0, 0, $data1[1], $data1[0], 2000+$data1[2] );
    echo $dia2 = mktime( 0, 0, 0, $data2[1], $data2[0], 2000+$data2[2] );

    echo ( $dia2 - $dia1 ) / 86400;