Always use a space between the dot and the concatenated parts to improve readability.

<?php
  $string = 'Foo' . $bar;
  $string = $bar . 'foo';
  $string = bar() . 'foo';
  $string = 'foo' . 'bar';
?>

When you concatenate simple variables, you can use double quotes and add the variable inside; otherwise, use single quotes.

<?php
  $string = "Foo $bar";
?>

When using the concatenating assignment operator ('.='), use a space on each side as with the assignment operator:

<?php
$string .= 'Foo';
$string .= $bar;
$string .= baz();
?>