ctype_alpha(sting) - true for [a-z or A-Z] checks whole string.
May also want to see the others:
ctype_alnum -- Check for alphanumeric character(s)
ctype_alpha -- Check for alphabetic character(s)
ctype_cntrl -- Check for control character(s)
ctype_digit -- Check for numeric character(s)
ctype_graph -- Check for any printable character(s) except space
ctype_lower -- Check for lowercase character(s)
ctype_print -- Check for printable character(s)
ctype_punct -- Check for any printable character which is not whitespace or an alphanumeric character
ctype_space -- Check for whitespace character(s)
ctype_upper -- Check for uppercase character(s)
ctype_xdigit -- Check for character(s) representing a hexadecimal digit
From that page --
CTYPE functions are always prefered, when possible, to Regular Expressions and, often, even to some equivalent str_*, is_* functions. This is because of the fact that CTYPE uses a native C library and thus processes signitificaly faster.
maxim at php dot net