$a .= "a";
$a .= "b";
$a .= "c";

$a .= "a"

$a = $a . "a"

$a .= "a"
// now $a == "a"
$a .= "b"
// now $a == "a" . "b" == "ab"
$a .= "c"
// now $a == "ab" . "c" == "abc"

$a = $a . "a"
$a .= "a"

$a .= "a";
$a .= "b";
$a .= "c";
echo $a;

$a .= "a";   // $a =$a . 'a' ie a= 'a'
$a .= "b"; //  $a =$a . 'b' ie a= 'ab'
$a .= "c"; // $a =$a . 'c' ie a= 'abc'