Form validation - security - php

Question

So I'm working on a register form and I have four fields: name, username, email and password. I pick up the values of these fields in jQuery and depending on if all the fields are filled, I pass them onto a PHP script via ajax. Is that safe for form validations? I was worried about data getting manipulated by the user.
Further on in the php script, I check if all the posted values have data in them, only then will I proceed onto doing some validations... The validations are the parts where I'm worried that it's not the best and there are many flaws with it.
$name = $_POST['name'];
$username = $_POST['username'];
$email = $_POST['email'];
$password = $_POST['password'];
if (!preg_match("/^[A-Za-z '-]+$/i", $name)) {
$errors = "Please enter a valid name.";
} else if (!preg_match("/^[A-Za-z]+\s[A-Za-z]+$/", $name)) {
$errors = "Please enter your first and last name.";
} else if (strlen($username) <= 3 || strlen($username) > 15) {
$errors = "Please pick a username between 4 - 15 characters. Be creative.";
} else if (!preg_match("/^[A-Za-z][A-Za-z0-9_.-]{4,15}$/", $username)) {
$errors = "Please pick an alphanumeric username between 4 - 15 characters.";
} else if (filter_var($email, FILTER_VALIDATE_EMAIL) === false) {
$errors = "Please enter a valid email.";
} else if (strlen($username )< 6 || strlen($username) > 32) {
$errors = "Your password must atleast be 6 characters.";
} else {
echo "valid";
}
Are these validation steps secure? Are there any loop holes that the user can manipulate the data? Also, I was hoping for the full name input, the user would input the first AND the last name. Are these good enough?
Thank you.

Answer 1

As you asked for review, pls read what you wrote here...
} else if (strlen($username )< 6 || strlen($username) > 32) {
$errors = "Your password must atleast be 6 characters."

Answer 2

Some times java script might not be the best solution since some users disables it. So consider that too.
And when inserting data to databases we should consider about 'sql injection', 'encryption methods' etc..
I Think you should read and study some more articles on the internet. Try these links.
http://www.sitepoint.com/form-validation-with-php/
http://www.w3resource.com/javascript/form/javascript-sample-registration-form-validation.php
http://www.hscripts.com/scripts/JavaScript/form-validation.php

Answer 3

I use this
$var = filter_var($_POST['xxx'], FILTER_SANITIZE_STRING);
If (preg_match('/^([a-zA-Z]){4,15}$/', $var)){ // must character lower or upper, minimal 4 and max 15 characters
$con1 = true;
} else{
$con2 = false;
}
// $cond2
// $cond3
// $cond4
// CHECK ALL CONDITION BEFORE INSERT
If (($cond1 && $cond2 && $cond3 && $cond4) == true){
(with prepared statement)
$sql = Insert into;
} else{
Header("Location: "); // if you redirect user to another page
And echo the errors with ajax
}
Hope you can consider this