One of the mysteries of PHP's echo function is the supposed equal treatment of multiple strings separated by periods (.) vs. those separated by commas (,). Actually echo is a language construct, but I digress. In both cases echo appears to concatenate the string fragments and output the resulting string.
In actuality, the period is the real concatenation operator in PHP. The comma on the other hand signifies echo's ability to accept variable-length arguments. Judging by Google search, most people just accept the fact that they can use either periods or commas with the echo function to get the job done.
But there's a subtle difference that's mostly overlooked because it rarely mucks up the results. Take a look at the two code lines below. You might expect to see 12 for both cases, but that is not so.
The reason is that with periods, some or all expressions are evaluated first and the results are concatenated. Then echo outputs the result after all fragments are concatenated. With commas echo walks the argument list, evaluating expressions and spitting out the results as it goes along.