How can special characters be removed using PHP?

Special characters can be removed using PHP by using the `preg_replace` function with a regular expression pattern that matches the special characters to be removed. This function can replace the matched special characters with an empty string, effectively removing them from the input string.

$input_string = "This is a string with special characters!@#";
$clean_string = preg_replace('/[^A-Za-z0-9 ]/', '', $input_string);
echo $clean_string;