PHP for date and email address and user name validation instance

PHP Tutorial Date mailbox Address User name authentication instance
<form id= "Form1" Name= "Form1" method= "Post" action= ""
  <label>name
  <input name= "name" type= "text" id= "name"/>
  </label>
  <p>
     <label>birthday
    <input name= "Birthday" type= "text" id= "Birthday"/>
    </label>
  </p>
  <P>
    <label> Email
    <input name= "Email" type= "text" id= "email"/>
    </label>
  </P>
  <p>
    <label>
    <input type= "Submit" Name= "submit" value= "submit"/>
    </label>
  </P>
</form
<?php
//This function is mainly used to check the error of the submitted form data (Validate)
//The actual application of user input data inspection (validate) and filter (filters) are involved in the program security, very important, Essential
//in writing the actual application will not put the $_post directly in the function, here is a palliative solution, I hope you can correct the

if ($_post)
{
 form_error ()
}
Function Form_error () {
    $_post[' name ' = Trim (strip_tags ($_post[' name '));//This is actually filter
    $len _name = strlen ($_post[' name '));
    if ($len _name > | | $len _name < 2) {
        $msg = ' Name length must be greater than 2 less than 30<br/> ';
   }
    $date = explode ('-', $_post[' birthday '));
    if (sizeof ($date)!= 3) {
        $msg. = ' Date format error <BR/ > ';
   } else {
        if (!checkdate ($date [1], $date [2], $ Date[0]) {
            $msg. = ' Incorrect date <br/> ';
       }
   }
    if (!eregi ("^[a-z ' 0-9]+ ([. _-][a-z ' 0-9]+) *@ ([a-z0-9]+ ([. _-][a-z0-9]+)] +$ ", $_post[' EMAIl ']) {
        $msg. = ' mailbox format error ';
   }
     return $msg;
}