What common mistake is highlighted in the PHP code snippet provided?

The common mistake in the provided PHP code snippet is the incorrect use of the concatenation operator within the double quotes. In PHP, when variables are placed inside double quotes, they are automatically interpolated, so there is no need to use the concatenation operator. To fix this issue, the concatenation operator should be removed.

// Incorrect code
$name = "Alice";
echo "Hello, ".$name."!";

// Corrected code
$name = "Alice";
echo "Hello, $name!";