(PHP 4 >= 4.0.4, PHP 5, PHP 7)
ctype_alpha — Check for alphabetic character(s)
$text
   )
   Checks if all of the characters in the provided string,
   text, are alphabetic.
   In the standard C locale letters are just 
   [A-Za-z] and ctype_alpha() is 
   equivalent to (ctype_upper($text) || ctype_lower($text))
   if $text is just a single character, but other languages have letters that 
   are considered neither upper nor lower case.
  
textThe tested string.
   Returns TRUE if every character in text is 
   a letter from the current locale, FALSE otherwise.
  
Example #1 A ctype_alpha() example (using the default locale)
<?php
$strings = array('KjgWZC', 'arf12');
foreach ($strings as $testcase) {
    if (ctype_alpha($testcase)) {
        echo "The string $testcase consists of all letters.\n";
    } else {
        echo "The string $testcase does not consist of all letters.\n";
    }
}
?>
The above example will output:
The string KjgWZC consists of all letters. The string arf12 does not consist of all letters.
Note:
If an integer between -128 and 255 inclusive is provided, it is interpreted as the ASCII value of a single character (negative values have 256 added in order to allow characters in the Extended ASCII range). Any other integer is interpreted as a string containing the decimal digits of the integer.