when i place this code:
<?php
$a=strip_tags(trim($_POST[roll_no]));
if ($a=="")
{
echo "<B><font color='red' size='3'><CENTER>YOU MUST ENTER ROLL NUMBER</CENTER></B></FONT>";
}
?>

IT WORKS FINE,
BUT WHEN I ADD THIS CODE:

<?php
$a=strip_tags(trim($_POST[roll_no]));
if ($a=="")
{
echo "<B><font color='red' size='3'><CENTER>YOU MUST ENTER ROLL NUMBER</CENTER></B></FONT>";
{
if ($a<>ereg("^[0-9]")
{
echo "enter number only";
}
}
}
?>
IT SAYS PARSE ERROR UNEXPECTED '{' ON LINE '8', CAN ANYONE HELP ME,WHERE I DID MISTAKE

you havent closed the first IF statement's curly brackets. After the line "echo "<b>..." change the open curly brackets to closed.

You might be after an "elseif" statement in which case it should look like this:

PHP Code:

if( X == Y )
{
//do something
}
elseif( X == Z )
{
//do something else
}
else
{
//do another thing
} 


Your conditionals are not correctly braced and your ereg does not have the correct parameters. http://uk3.php.net/ereg

Try

PHP Code:

$a=strip_tags(trim($_POST['roll_no']));
if ($a=="")
{
echo "<B><font color='red' size='3'><CENTER>YOU MUST ENTER ROLL NUMBER</CENTER></B></FONT>";
}
if (ereg('[^0-9]', $a)) {

echo "You entered $a , please enter number only";
} 


BTW, the code you posted doesn't really need any curly braces. But I advise you to indent it properly, making it more readable.