✔ slideToggle() nur auf dem gerade geklickten Element verwenden

<td><span class="historie">?</span></td>

<tr><td colspan="7" class="historie-inner">test</td></tr>

$(document).ready(function(){
    $("span.historie").click(function () {
        $("td.historie-inner").slideToggle("fast");
    });
})

<table border="1" rules="rows">

    <thead><tr>
        
        <th>&nbsp;</th>
        
        <th colspan="3">Inhaber</th>
    
        <th>Aktuell bei:</th>
    
        <th>Schlüssel-Nr.:</th>
    
        <th>Verlauf</th>
        
    </thead></tr>

<?php

if ($anzahl_inhaber > 0) {
    $i=0;
    while ($inhaber = mysql_fetch_object($abfrage_inhaber)) {
        $i++;
        //Benutzername des Owners holen
        $abfrage_benutzername              = mysql_query("SELECT benutzername FROM benutzer WHERE id = '" . $inhaber->benutzer_id . "' ");
        $benutzername                      = mysql_fetch_object($abfrage_benutzername);
        
        //Schlüssel holen
        $query_schluessel                  = "SELECT * FROM schluessel WHERE key_owner_id = '" . $inhaber->benutzer_id . "'";
        $abfrage_schluessel                = mysql_query($query_schluessel);
        $schluessel                        = mysql_fetch_object($abfrage_schluessel);
        
        //Current_location
        $query_current_location            = "SELECT * FROM current_location WHERE key_id = '" . $schluessel->key_id . "' ORDER BY start_date DESC ";
        $abfrage_current_location          = mysql_query($query_current_location);
        $current_location                  = mysql_fetch_object($abfrage_current_location);
        
        //Benutzername von Current_Person (wenn nicht owner)
        $query_current_location_benutzer   = "SELECT benutzername FROM benutzer WHERE id = '" . $current_location->current_person . "'";
        $abfrage_current_location_benutzer = mysql_query($query_current_location_benutzer);
        $current_location_benutzer         = mysql_fetch_object($abfrage_current_location_benutzer);
        
        //Seit: beim anderen user
        $query_current_person_date         = "SELECT start_date FROM current_location WHERE current_person = '" . $current_location->current_person . "'";
        $abfrage_current_person_date       = mysql_query($query_current_person_date);
        $current_person_date               = mysql_fetch_object($abfrage_current_person_date);
        
        $query_mitarbeiter                 = "SELECT benutzername FROM benutzer";
        $abfrage_mitarbeiter               = mysql_query($query_mitarbeiter);
        
?>

    <tbody><tr>

        <td>
            <?php if ($schluessel->key_owner_id == $current_location->current_person) {?>
            <a href="#"><img src="images/schluessel-bei-inhaber.png" alt="Schlüssel bei Inhaber" title="Schlüssel bei Inhaber" /></a>
            <?php }else{?>
            <a href="#"><img src="images/schluessel-bei-anderen.png" alt="Schlüssel verliehen" title="Schlüssel verliehen" /></a>
            <?php }?>
        </td>
    
        <td><?php echo $benutzername->benutzername; ?></td>
        
        <td>
            <div class="check">    
                <input type="checkbox" id="check<?php echo $i;?>" <?php if ($schluessel->bks == 1) { echo 'checked="checked"'; } ?> DISABLED/><label for="check<?php echo $i;?>">bks</label>
                <input type="checkbox" id="check<?php echo $i = $i+1;?>" <?php if ($schluessel->keso == 1) { echo 'checked="checked"'; } ?> DISABLED/><label for="check<?php echo $i;?>">keso</label>
            </div>
        
        </td>
        
        <td><a class="edit" href="http://localhost/key/liste.php?key_id=<?php echo $schluessel->key_id;?>"><img src="images/datensatz-bearbeiten.png" alt="Datensatz bearbeiten" title="Datensatz bearbeiten" /></a></td>
        
        <td>
        
            <?php
    
            if ($current_location->current_person != $inhaber->benutzer_id) {
            
            ?>
            
                <ul>
                    <li>bei: <?php echo $current_location_benutzer->benutzername; ?></li>
                    <li>seit: <?php echo $current_person_date->start_date; ?></li>
                </ul>
            
            <?php
            
            }
            
            ?>
        
        </td>
        
        <td align="center"><?php echo $schluessel->key_id; ?></td> <!-- Hier das öffnende Element -->
        
        <td><span class="historie">?</span></td>

        
    
    </tr></tbody>
    
    <tr>
        <td colspan="7" class="historie-inner"> <!-- Hier das versteckte Element -->
            <p>
            <table style="font-size:11px; width: 500px;" cellpadding="0" cellspacing="0" border="0">
                <tr>
                    <td width="100"><strong>Benutzer:</strong></td>
                    <td width="150"><strong>angenommen:</strong></td>
                    <td><strong>abgegeben:</strong></td>
                </tr>
                <?php
                $query_historie = "SELECT * FROM current_location WHERE key_id = '".$schluessel->key_id."' ORDER BY start_date DESC";
                $abfrage_historie = mysql_query($query_historie);
                while($historie = mysql_fetch_object($abfrage_historie)){
                
                $query_name = "SELECT benutzername FROM benutzer WHERE id = '".$historie->current_person."'";
                $abfrage_name = mysql_query($query_name);
                $name = mysql_fetch_object($abfrage_name);
                    
                    if ($historie->current_person != $inhaber->benutzer_id) {
                ?>
                <tr <?php if($historie->end_date != NULL){ echo "style='color: #555;'"; }?> onMouseOver="this.style.backgroundColor='#666';" onMouseOut="this.style.backgroundColor='#333';">
                    <td><?php echo $name->benutzername;?></td>
                    <td><?php echo $historie->start_date;?></td>
                    <td><?php if ($historie->end_date == NULL){ echo "<span class='danger'>hat ihn noch!</span>"; } else { echo "<img src='images/abgegeben.png' alt='abgegeben' title='abgegeben' />".$historie->end_date; } ?></td>
                </tr>    
                <?php
                    
                    }
                
                }
                
                ?>
                
            </table>
            </p>
        </td>
    </tr>
<?php

    }
    
?>

</table>

var historieAll = $('.historie');
var historieInnerAll = $('.historie-inner');
 
historieAll.click(function() {
    var index = historieAll.index(this);
    historieInnerAll.eq(index).slideToggle('fast');
});