How can the issue of "Undefined variable: res" be resolved without turning off error reporting in PHP?

To resolve the issue of "Undefined variable: res" in PHP without turning off error reporting, you can initialize the variable before using it to avoid the error message. This can be done by setting the variable to null or an empty value before assigning any actual value to it.

$res = null; // Initializing the variable
// Other code that may assign a value to $res
echo $res; // Using the variable without causing an error