question about database injection

i am helping a friend to build a forum website which uses php and
mysql database.

i am working on the registeration page for the forum website and its
validation. i am using php 5.2.5

i am able to validate and do other tasks, however i really need help
as i am stuck with regards to database injection.

please answer the following questions. any help will be greatly
appreciated.

1. USER NAME VALIDATION

username = eregi("^[a-zA-Z0-9_ ]+$", $username)

with the above validation, a user can enter letters uppercase,
lowercase and numbers and underscore with spaces ONLY ex= 9abc_def OR
_abc123 = this IS INCORRECT

however i would like the username to be Letters First(upper or
lowercase), followed by numbers and underscore and spaces in the
username.

ex= abcd1234 OR ABcd1234 OR Ab_12 OR ab 12_cd OR 123456 OR 123abc =
this IS CORRECT

i have used with preg_match as => if( $username == "" || !preg_match('/
^[a-zA-Z0-9_]+$/x', $username) ) however its the same as eregi

QUESTION = how can i rewrite username = eregi("^[a-zA-Z0-9_ ]+$",
$username) to match the following requirement.
username = abcd1234 OR ABcd1234 OR Ab_12 OR ab 12_cd OR 123456 OR
123abc
also with eregi("^[a-zA-Z0-9_ ]+$", $username) as there is a space if
a user has more than 1 space ex= "ab cd 12" it is still accepting is
there a way to restrict to ONE space only
ex = "ab cd12"

2. USING mysql_real_escape_string() METHOD

i am able to validate username, first name, phone numbers based on
preg_match for these individual ones, however the form consists of
some optional fields which i am not validating so if a user enters
invalid characters in these optional fields i need to protect from sql
injection, presently my code for mysql_real_escape_string() is as
follows and the special characters
are still appearing in the database. i have not used
mysql_real_escape_string() before so i guess i am missing something

$conn = mysql_connect($hostname, $user, $dbpassword);

$insertquery = sprintf("INSERT INTO tablename (`username`, `password`,
`firstname`) VALUES ('%s', '%s', '%s')",

mysql_real_escape_string($username, $conn),
mysql_real_escape_string($password, $conn),
mysql_real_escape_string($firstname, $conn));

should i be checking for if(get_magic_quotes_gpc()) { } first.

NOTE = by using this mysql_real_escape_string() method php should NOT
add slashes or other characters if this happens then the username will
be stored in the table differently ex= john`smith instead it should be
johnsmith the slashes can be done for other fields like firstname etc
as this username and password will be used by a user to login to the
forum

please advice about the procedure for mysql_real_escape_string()
method

3. QUESTION ABOUT SQL INJECTION

presently if i enter special characters in the form these values are
being inserted to the database as it is which is not good. out of the
following methods
htmlentities(), addslashes(), trim(), mysql-real-escape-string() which
is the best method to use to avoid sql injection
i think mysql-real-escape-string() is the best method.
NOTE = in my php settings magic_quotes_gpc is ON,
magic_quotes_runtime is OFF, magic_quotes_sybase is OFF

4. STORING PASSWORDS

as part of the registration for the forum the username and password
that the user enters in the registration page will be used as their
username and password to login to the forum. presently when i execute
the sql insert statement along with other fields for the registration
page the value of the password stored in the mysql table is the actual
characters that a user entered in the form. in the form the element is
defined as <input type="password" name="password"> however in the
table the password is stored as the actual characters the user entered
in the form. is this a right way of storing the password field from
the form.

NOTE = i believe with websites that are forum based using php and
mysql, there is a way to pass information to the php file which will
automatically pick up the username and password from the table that i
have created where i am storing the username and password.

Please comment on storing the password in mysql table and how i can
find the php file to which i can pass the value of username and
password as a variable by using a function to that php and by
including that php file in which i am processing the registration
form.

Thanks a lot for reading my post. Any help will be greatly
appreciated.
.

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