What are some potential pitfalls when using PHP to handle form input and output, especially when dealing with variables within strings?

When dealing with variables within strings in PHP, a potential pitfall is forgetting to properly concatenate the variables within the string. This can lead to syntax errors or unexpected output. To avoid this issue, always use proper concatenation techniques such as using double quotes around the string and using the concatenation operator (.) to insert variables.

// Incorrect way of handling variables within strings
$name = "John";
echo "Hello, $name!"; // This may lead to syntax errors or unexpected output

// Correct way of handling variables within strings
$name = "John";
echo "Hello, " . $name . "!"; // This ensures proper concatenation of the variable within the string