<?php

$fileFormatSample = '02.08.2012 83.7
04.08.2012 82.9
09.08.2012 82.1
26.08.2012 81.5
03.08.2012 83.0
10.08.2012 81.0
13.08.2012 83.7
22.08.2012 83.4
05.08.2012 82.2
25.08.2012 82.4
23.08.2012 82.5
31.08.2012 81.4';

function sortFile($file)
{
    $content = file_get_contents($file);
    $lines = preg_split("/[\r\n]+/", $content);

    $data = [];
    foreach ($lines as $line) {
        $line = explode(' ', $line);
        $ts = DateTime::createFromFormat('d.m.Y', $line[0])->getTimestamp();
        $data[$ts] = $line[1];
    }
    ksort($data);

    $finalData = [];
    foreach ($data as $time => $weight) {
        $finalData[Date('d.m.Y', $time)] = $weight;
    }

    file_put_contents($file, implode("\n", $finalData));
}