True allergies to cow's milk are relatively uncommon. Only 2 to 3 percent of children are allergic to milk, according to the AAP, and almost all of them outgrow it by age 3. (Learn the difference between a milk allergy and lactose intolerance .).
The underscore [ _ ] (also called underline, underbar, low line, or low dash) is a character that originally appeared on the typewriter and was primarily used to underline words.
ctype_alnum() is a godsend for quick and easy username/data filtering when used in conjunction with str_replace(). Let's say your usernames have dash(-) and underscore(_) allowable and alphanumeric digits as well. It is also important to note that the behavior of `ctype_alnum` differs according to the operating system.