Using PHP's echo, when does the difference between . and , matter?

Question

I noticed recently that there are two ways to print multiple statements in PHP.

echo $a.$b; // Echo's $a and $b conjoined, and
echo $a,$b; // Echo's $a and echo's $b.

Is there any point in time where the difference between these two syntaxes matters?

Answer 1

Realistically, no.

echo $a.$b first concatenates $a and $b into a new string, then passes it as a parameter to echo, which prints it out.

echo $a,$b gives two parameters to echo, which will print both out.

The latter is slightly more efficient. Not in any way that you would normally notice though.

There is a difference in how it is evaluated. echo $a, $b is like writing echo $a; echo $b;, two separate calls. $b will be evaluated after $a is echo'd. This can make a difference if your arguments are function calls which themselves echo something, but again, in practice this should be irrelevant, since it's bad practice.

Answer 2

There is a difference that is good to note. With the second synthax, the first parameter will still output even if the second one causes an error.

<?php
echo 1, error(); // This outputs 1, then it displays an error 
?>

While the first one won't echo the first part.

<?php
echo '1' . error(); // Only displays an error
?>

When you separate your parameter with a comma, it will echo them one after the other. When you use the dot operator, it will concatenate them into a string and then it will echo it.