preg_replace problem

[Benjamin Adams <badams@xxxxxxxxxxxxxx>]

$file = dog.txt;
$today = date("Ymd");

function incDate($new, $date){
        //$date = settype('int');
        return $new.($date++);
}

$getOldValue = parse_ini_file($file, 1);
$newValue = $getOldValue[$today] + 1;
$oldDate = $today . " = ". $newValue;
$newDate = preg_replace('/(\d+\s\=\s)(\d+)/ie', 'incDate("$1","$2")',  
$oldDate);
file_put_contents($file, $newDate . "\n");

This works with one file but with multi lines I'm having trouble:
if the file has:
20060301 = 34
20060302 = 3
the file after script will be:
20060302 = 4

I want it to preserve the previous lines
so output should be:
20060301 = 34
20060302 = 4

Help would be great, thanks
Ben