What common mistake did the user make in the PHP code snippet provided?

The common mistake in the provided PHP code snippet is the incorrect use of concatenation within the echo statement. Instead of using the '.' operator to concatenate strings, the user mistakenly used the '+' operator which is not valid in PHP for string concatenation. To fix this issue, the user should replace the '+' operator with the '.' operator to concatenate the strings properly.

// Incorrect code snippet
$name = "John";
echo "Hello, " + $name;

// Corrected code snippet
$name = "John";
echo "Hello, " . $name;