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I would still like to know how to tell whether someone has entered anything other than text.
For example: Cardholder's name on a credit card should be text only, how can I make sure text (a-z) is entered (allowing whitespace) and not anything else ?
for simple validation rules, you can use ctype_alpha() (letters only), ctype_alnum() (letters and numbers), and is_numeric() (numbers only).
For more complex validation rules, such as a credit card your best bet is creating a regular expression with preg_match(), however there are many solid regular expressions already made if you poke around on google.