PHP strtotime奇怪的日期问题

I'm trying to store some dates in my datebase (MySQL). But I got a strange conversion error: This is my piece of PHP code:

$fechanac=mysql_real_escape_string(($_POST["fechanac"]));
echo "<h1>{$fechanac}</h1>";
$fechanac=date('Y-m-d', strtotime(str_replace('/', '-', $fechanac)));
echo "<h1>{$fechanac}</h1>";exit();

See the following three example, trying with (01/01/1900, 01/01/1901 and 01/01/1902).

OUTPUT:

01/01/1900

1970-01-01

01/01/1901

1970-01-01

01/01/1902

1902-01-01

Somebody know why happens this? And how to fix it? I need to insert in my DB, possible dates of living persons. Thanks for reading.

Date stored!

$fechanac=mysql_real_escape_string(($_POST["fechanac"]));
echo "<h1>{$fechanac}</h1>";
$datearray=explode("/",$fechanac);
$fechanac="{$datearray[2]}-{$datearray[1]}-{$datearray[0]}";

I know it's and old post, but maybe this late answer can help others:

The DateTime class correctly manages dates older than 1970. So forget the use of date() and evolve to DateTime:

$fechanac=mysql_real_escape_string(($_POST["fechanac"]));
echo "<h1>{$fechanac}</h1>";

$fechanac=(new DateTime($fechanac))->format('Y-m-d');

echo "<h1>{$fechanac}</h1>";exit();