What does the ‘period’ character (.) mean if used in the middle of a PHP string?

Here is some example code:

$headers = 'From: [e-mail]' . "rn" .
    'Reply-To: [e-mail]' . "rn" .
    'X-Mailer: PHP/' . phpversion();

What does the period character do in the middle of each piece of the string?

For example,

"blabla" . "blabla" . "blablalba";

Answer

This operator is used to combine strings.

EDIT

Well, to be more specific if a value is not a string, it has to be converted to one. See Converting to a string for a bit more detail.

Unfortunately it’s sometimes mis-used to the point that things become harder to read. Here are okay uses:

echo "This is the result of the function: " . myfunction();

Here we’re combining the output of a function. This is okay because we don’t have a way to do this using the standard inline string syntax. A few ways to improperly use this:

echo "The result is: " . $result;

Here, you have a variable called $result which we can inline in the string instead:

echo "The result is: $result";

Another hard to catch mis-use is this:

echo "The results are: " . $myarray['myvalue'] . " and " . $class->property;

This is a bit tricky if you don’t know about the {} escape sequence for inlining variables:

echo "The results are: {$myarray['myvalue']} and {$class->property}";

About the example cited:

$headers = 'From: [e-mail]' . "rn" .
    'Reply-To: [e-mail]' . "rn" .
    'X-Mailer: PHP/' . phpversion();

This is a bit tricker, because if we don’t use the concatenation operator, we might send out a newline by accident, so this forces lines to end in “rn” instead. I would consider this a more unusual case due to restrictions of email headers.

Remember, these concatenation operators break out of the string, making things a bit harder to read, so only use them when necessary.