<?php
$input_string = "05.07.2000,00:00:00,0,6856868,0,7779905,0,6856868,0,6856868,42539730";
 
    $parts = explode(",",$input_string);
    $head = array_splice($parts,0,2);
    $tail = array_chunk($parts,2);
    $result = implode(",",$head) . "," . implode(",",
        array_map(function($p) {return implode(".",$p);}, $tail));
 
echo $result;
 
echo "\n\n";
 
$expected = "05.07.2000,00:00:00,0.6856868,0.7779905,0.6856868,0.6856868,42539730";
var_dump($result === $expected);

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