$num = 786733;
$addFlag = 0;

if(length($num) >= 3 ) {

    $var = substr($num,-3,1);
    $var += 1;
    if($var1 == 10) { 
          $addFlag = 1;
          $var1 = 0;
    }
    $var .='00';

    $restN = length($num) - 3;
    $var1 = substr($num,0,$restN);
    $var1 +=1 if $addFlag;

    print $var1.$var;
}
output: 786800;