Form validation using PHP

I want to validate my form so ALL of the fields are required. If a field is NOT inserted or left blank it will display an error message AFTER submission. Could anyone help?

Form

<html>  
<head>  
<title>Form Input Data</title> 
</head>
<table>  
<body><table border="1">
<table bgcolor="lightblue"></body>

     <form method="post" action="insert_ac.php"> 
    <br>
<tr><td align="left"><strong>Nurse Information</strong></td></tr>
<tr>
<td><font color="red">Please select your name</font></td>
</tr>
<tr>
<td>Fullname</td>
<td><select name="valuelist">;
<option value="valuelist" name="nurse_name" value='<?php echo $nurse_name;  ?>'></option>
<?php
$value=$_POST ["valuelist"];
$con = mysql_connect("localhost","root","") or die('Could not connect:'.mysql_error());
mysql_select_db("a&e", $con) or die('Could not select database.');

$fetch_nurse_name = mysql_query("SELECT DISTINCT Fullname FROM nurse");

while($throw_nurse_name = mysql_fetch_array($fetch_nurse_name)) {
echo '<option   value=\"'.$throw_nurse_name[0].'">'.$throw_nurse_name[0].'</option>';
 }
 echo "</select>";

?>
</td>
</tr>
<tr>
<td>Please register name here:</td>
<tr>  

        <td>Fullname</td>

       <td><input type="text" name="nurse_forename" size="30"> </td>

     </tr>
 </tr>

Solution

I would do something like this:

$req = ['field1', 'field2', 'field...'];
$status = true;
foreach ($req as $field) {
    if (empty($_POST[$field])) {
        echo 'Field ' . $field . ' is empty';
        $status = false;
    }
}
if ($status) {
    // ok
} else {
    // not okay!
}

You create an array ($req), with all field names and loop over them. Check every field against empty() (check the php manual for this function).

Here is a better (and mostly) correct HTML snippet... Please indent properly and read any HTML tutorial for well formed code. Your HTML is **.

<?php

$value=$_POST["valuelist"];
$con = mysql_connect("localhost","root","") or die('Could not connect:'.mysql_error());
mysql_select_db("a&e", $con) or die('Could not select database.');

$fetch_nurse_name = mysql_query("SELECT DISTINCT Fullname FROM nurse");

?>
<html>
<head>
    <title>Form Input Data</title> 
</head>
<body>

    <form method="post" action="insert_ac.php"> 

    <table border="1" bgcolor="lightblue">
        <tr>
            <td align="left"><strong>Nurse Information</strong></td>
        </tr>
        <tr>
            <td><font color="red">Please select your name</font></td>
        </tr>
        <tr>
            <td>Fullname</td>
            <td>
                <select name="valuelist">
                    <option value="valuelist" value="<?php echo $nurse_name;  ?>"></option>
                    <?php

                    while($throw_nurse_name = mysql_fetch_array($fetch_nurse_name)) {
                        echo '<option value="'.$throw_nurse_name[0].'">'.$throw_nurse_name[0].'</option>';
                    }
                    ?>
                </select>
            </td>
        </tr>
        <tr>
            <td>Please register name here:</td>
        </tr>
        <tr>  
            <td>Fullname</td>
            <td><input type="text" name="nurse_forename" size="30"> </td>
        </tr>
    </table>
    </form>
</body>
</html>

If you have only the two given fields, this would do it:

$status = false;
$name = '';

if (!empty($_POST['nurse_forename'])) {
    $name = $_POST['nurse_forename'];
    $status = true;

} elseif (!empty($_POST['valuelist'])) {
    $name = $_POST['valuelist'];
    $status = true;

} else {

    $status = false;
    // none of nurse_forname OR valuelist is filled
    // abort.
}