Registration system -- need some help!

Hi folks, I'm building a registration system for a site for user accounts. I made the form, and it adds the info to a MySQL database. Anyways, I'm concerned with security and validity. Here are some things that I need to know how to do:

limit characters -- only specific characters can be used, for example only the characters A-Z can be used for entering a name.

check for valid email -- emails must follow the standard format.

character count limit -- if the character count is greater than a certain amount, the entered info is invalid, so a user can't add a 10000 character username for example.

these are the major things that I need help with. Any help, or even a point in the right direction, will be greatly appreciated. Thanks!

 

functions:

<?php

function isValidEmail($email) {
    // First, we check that there's one @ symbol, 
    // and that the lengths are right.
    if (!ereg("^[^@]{1,64}@[^@]{1,255}$", $email)) {
        // Email invalid because wrong number of characters 
        // in one section or wrong number of @ symbols.
        return false;
    }
    
    // Split it into sections to make life easier
    $email_array = explode("@", $email);
    $local_array = explode(".", $email_array[0]);
    for ($i = 0; $i < sizeof($local_array); $i++) {
        if (!ereg("^(([A-Za-z0-9!#$%&'*+/=?^_`{|}~-][A-Za-z0-9!#$%&↪'*+/=?^_`{|}~\.-]{0,63})|(\"[^(\\|\")]{0,62}\"))$", $local_array[$i])) {
            return false;
        }
    }
    
    // Check if domain is IP. If not, 
    // it should be valid domain name
    if (!ereg("^\[?[0-9\.]+\]?$", $email_array[1])) {
        $domain_array = explode(".", $email_array[1]);
        if (sizeof($domain_array) < 2) {
            return false; // Not enough parts to domain
        }
        
        for ($i = 0; $i < sizeof($domain_array); $i++) {
            if (!ereg("^(([A-Za-z0-9][A-Za-z0-9-]{0,61}[A-Za-z0-9])| ↪([A-Za-z0-9]+))$", $domain_array[$i])) {
                return false;
            }
        }
    }
    return true;
}

// ---------------------------------------------

function isOnlyLetters($var)
{
    return (preg_match("/^[a-zA-Z]+$/", $var)) ? true : false;
}

// ---------------------------------------------

function isValidLength($var, $length)
{
    return (strlen($var) <= $length) ? true : false;
}

PHP:

Use:

$email = 'me@me.com';

if (isValidEmail($email)) {
    echo 'valid';
} else {
    echo 'invalid';
}



$field = 'testing';

if (isOnlyLetters($field)) {
    echo 'only letters';
} else {
    echo 'not just letters!';
}



$field = '12345678';

if (isValidLength($field, 8)) {
    echo 'valid length';
} else {
    echo 'invalid length';
}

PHP:

 

Thanks a lot

 

can we use below?

function isOnlyLetters ($string) {
     return ( ctype_alpha ( $string ) ) ? true : false; // PHP 4 >= 4.0.4, PHP 5
}

Code (PHP):

OR just use the PHP function ctype_alpha($string) lonely, don't have to call an external function

$field1 = 'testingazAZ';
if (ctype_alpha($field1)) {    echo $field1.' : only letters<br>';} 
else {    echo $field1.' : not just letters<br>';}

Code (PHP):