solusi 2020-01

<ul>
<?php
$values = array();
$values[] = array('example'=>'example1','date'=>'2020-01-25');
$values[] = array('example'=>'example2','date'=>'2020-02-25');
$values[] = array('example'=>'example3','date'=>'2020-03-25');

$previous = null;
$pesan=[];
foreach($values as $i=>$gunawan)
{
    //echo "<li>".$example['example']." - ".$example['date'];
    $sementara=['date'=>$gunawan['date'], 'selisih'=>0];
        $pesan[]=$sementara;
/*
        if ($previous !== null) {
        $diff = strtotime($example['date'])-strtotime($previous);
        echo " - ".($diff/(60*60*24))." Hari";
    }

    $previous = $example['date'];
    echo "</li>\n";
*/
        if($i!=0){
        $selisih=strtotime($gunawan['date'])-strtotime($pesan[$i-1]);
        $selisih_hari = $selisih / (60*60*24) ;
        $pesan[$i-1]=$selisih_hari;
        }
}


foreach($pesan as $row){
    echo "<li>{$row[date]} - {$row[selisih]}</li>";

}
?>
</ul>