The strings are incremented per character and each character position can be one of 3 modes:1. [A-Z] uppercase2. [a-z] lowercase3. [0-9] decimalyou can mix any combination of the modes and (at least in right to left languages like english) it always increments from the right overflowing leftwardsthe mode/type of character that overflows remains the mode/type of the first (0 index) position.so: input 'zZ9' & 'aaA0' is returnedso: input 'Z9z' & 'AA0a' is returnedso: input '9zZ' & '10aA' is returnedExample:<?php$str = 'zZ9'; // overflows in lowercaseecho $str = str_increment($str).PHP_EOL; // aaA0$str = 'Z9z'; // overflows in uppercaseecho $str = str_increment($str).PHP_EOL; // AA0a$str = '9zZ'; // overflows in decimalecho ($str = str_increment($str)); // 10aA?>