check for special characters?

[android6011]

is there a function that checks for special charactars? I want an input of only numbers and letters for a username so Im not sure the best mthod for making sure only numbers or letters have been entered

[BlueSkyIS]

if (ereg('^[A-Za-z0-9]+$', $user_name)) {
echo "$user_name IS all numbers and letters.";
} else {
echo "$user_name IS NOT all numbers and letters.";	
}

[android6011]

thank you