MySQL-Error：where子句中的未知列

<div class="ui-widget">

<form method="POST">
<?php

if(isset($_POST['search'])){

$connection = mysql_connect('localhost', 'root', '1234');
mysql_select_db('hoppers');

    $query = "SELECT user_name FROM users WHERE {$_POST['username']} = user_name";
    $result = mysql_query($query) or die ("MySQL-Error: " . mysql_error());
    $row = mysql_fetch_array($result);


if($row = TRUE){

        $present = $row['user_name'];
        echo $present;

     }else{
            echo "This User does not exist!";
        }
}
?>

<label for="tags">Username: </label>
<input type="text" id="tags" name = "username">
<input type="submit" name="search" value="Suchen!">
</form>
</div>

So I'm always getting the this error-message: MySQL-Error: Unknown column 'dompol171' in 'where clause'.
So it is comparing the username that I entered with the column "user_name" itself. I keep freaking out...since I have no clue how to compare it in a different way!

Try using :

 $query = "SELECT user_name FROM users WHERE user_name = '{$_POST['username']}'";

Query needs slight adjustment, from:

$query = "SELECT user_name FROM users WHERE {$_POST['username']} = user_name";

to:

$query = "SELECT user_name FROM users WHERE user_name = '{$_POST['username']}'";

Wrap the value in single quotes.

This updated PHP will validate the data passed back:

<?php
if(isset($_POST['search'])){

$connection = mysql_connect('localhost', 'root', '1234');
mysql_select_db('hoppers');

$query = "SELECT user_name FROM users WHERE user_name = '{$_POST['username']}'";
$result = mysql_query($query) or die ("MySQL-Error: " . mysql_error());

// count # of rows returned by DB query
$num_users = mysql_num_rows( $result );

// If more than 0
if ( $num_users > 0 ){
    // Success!
    $row = mysql_fetch_array($result);
    var_dump( $row );
} else {
    // No results returned
    echo "This User does not exist!";
}


}

You have made a mistake in where clause , it's :

WHERE user_name = {$_POST['username']}";

The syntaxe is : Where "column" = "value"

You have your parameters backwards. Column name always goes before the value:

$query = "SELECT user_name FROM users WHERE user_name = {$_POST['username']}";

This isn't a comparison operator like in PHP where the operands can be on either side of the equation.

What is happening here is the value of $_POST['username'] is being considered a column since you didn't quote it i.e. set it as a string. So say the user name is musa, its trying to compare column musa to column user_name(and that column doesn't exist). It should be

 $query = "SELECT user_name FROM users WHERE 'some username' = user_name";

Try Using This:

$username = $_POST['username'];
$query = "SELECT user_name FROM users WHERE user_name='" . $username . "'";