错误:strpos()[function.strpos]:空分隔符

I got this littel problem here with this code

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

    1<input type="text" name="A" <br>
    2<input type="text" name="B" <br>
    3<input type="text" name="C" <br>
     <input type="submit" value="Submit"><br>

</form> 

and PHP

<?php

$a = $_POST['A'];  $b = $_POST['B'];  $c = $_POST['C'];
if(!isset($a , $b) || trim($a , $b ) == '' )
{
    echo "empty" ;
}
$szukaj = strpos($a,$b );

if ($szukaj === false ) {
    echo 'hehe ';
} else {
    echo 'hihi ';
}
?>

Icant deal with this error... Can somebody give some tip ,or advice... what i do wrong?

In your case you should do like this:

<?php
$a = $_POST['A'];  $b = $_POST['B'];  $c = $_POST['C'];
if(!empty($a) || empty($b) || trim($a) == '' || trim($b) == '') {
    echo "empty" ;
} else {
    $szukaj = strpos($a, $b);
    if ($szukaj === false ) {
        echo 'hehe ';
    } else {
        echo 'hihi ';
    }
}
?>

Your checking for empty were a bit off, and you have to use an else on the empty conditions to only run the strpos if they aren't empty.
I've also placed the other if in the else to be sure the $szukaj variable is set.

your html is wrong, I've tidied it up a bit for you

<form action="index.php" method="post">
    1<input type="text" name="A" /><br />
    2<input type="text" name="B" /><br />
    3<input type="text" name="C" /><br />
    <input type="submit" value="Submit" /><br />
</form>