Strings
http://us3.php.net/manual/en/book.strings.php
strlen
strcmp, strcasecmp
strspn, strcspn
strtolower, strtoupper, ucfirst, ucwords
nl2br, htmlentities, htmlspecialchars, strip_tags, strtr(get_html_translation_table
Regular Expressions
Regular Expression (POSIX Extended)
http://us3.php.net/manual/en/book.regex.php
Regular Expressions (Perl-Compatible)
http://us3.php.net/manual/en/book.regex.php
Alternatives for Regular Expression Functions
strtok, explode, implode
strpos, str_replace, strstr, substr, substr_count, substr_replace
trim, ltrim, rtrim, str_pad
count_chars, str_word_count
String Validation with PEAR Validate_US
http://pear.php.net/package/Validate_US