Passing multiple parameters to echo using commas (',')is not exactly identical to using the concatenation operator ('.'). There are two notable differences.

First, concatenation operators have much higher precedence. Referring to http://php.net/operators.precedence, there are many operators with lower precedence than concatenation, so it is a good idea to use the multi-argument form instead of passing concatenated strings.

<?php
echo "The sum is " . 1 | 2; // output: "2". Parentheses needed.
echo "The sum is ", 1 | 2; // output: "The sum is 3". Fine.
?>

Second, a slightly confusing phenomenon is that unlike passing arguments to functions, the values are evaluated one by one.

<?php
function f($arg){
  var_dump($arg);
  return $arg;
}
echo "Foo" . f("bar") . "Foo";
echo "\n\n";
echo "Foo", f("bar"), "Foo";
?>

The output would be:
string(3) "bar"FoobarFoo

Foostring(3) "bar"
barFoo

It would become a confusing bug for a script that uses blocking functions like sleep() as parameters:

<?php
while(true){
  echo "Loop start!\n", sleep(1);
}
?>

vs

<?php
while(true){
  echo "Loop started!\n" . sleep(1);
}
?>

With ',' the cursor stops at the beginning every newline, while with '.' the cursor stops after the 0 in the beginning every line (because sleep() returns 0).