<?php
// You can use a variable $message, if anything goes wrong
// From beginning $message is empty
$message ='';
// test submitted fields here
if ( $some_error ) $message = 'some error message';
?>
<!-- Your HTML form part -->
<?php
// if NOT empty
if ( !empty( $message )) echo $message;
// else is empty
else echo '<b>Welcome! Fill in required fields, please.</b><br>';
?>
Fields marked <b>*</b> are required.
<form .......... >
<input .............> <b>*</b><br>
<input .............> <br>
<input .............> <b>*</b><br>
<input type='submit value='SUBMIT'>
</form>