php - how to handle mysql update error using ajax

one text

Solution:

Tweak the PHP & HTML so that the nesting is correct and assign a new dataset attribute to the button rather than the inline event handler.

<?php

    $root = realpath(str_replace('\\', '/', $_SERVER['DOCUMENT_ROOT']) ); 
    include ($root . '/insights/ss/onix.php'); 

    $result = mysqli_query($mysqli,"select * from notifications where seen = 0");
        if ($result){
            if($result->num_rows) {
                while($row = mysqli_fetch_assoc($result)){
?> 
    
<div class='alert alert-success alert-dismissible' role='alert' style='margin-left:-12px;'>
    <button type="button" class="close" data-id="<?=$row['id'];?>" data-dismiss="alert" aria-label="Close" style="float:left!important; border:0; background:none;">
        <span aria-hidden="true">&times;</span>
    </button>

    <strong>
        <span class="text-success" style="margin-top:-50px;">
            <i class='fa fa-check'></i>
            &nbsp;&nbsp;&nbsp;File has been moved successfully
        </span>
    </strong>
    <br>
    To confirm reading this message please press X button 
</div>

<?php
           }
        }
    }
?>

Use an externally registered event handler and why not use the fetch api ~ appears slightly shorter and is a better api moving forwards.

<script>
    function updateId(e){
        e.stopPropagation();
        let id=e.target!=e.currentTarget ? e.target.parentNode.dataset.id : e.target.dataset.id;
        fetch( 'dismisssuccess.php?id='+id )
            .then(r=>r.text())
            .then(text=>console.log(text))
    }
    document.querySelectorAll('div[role="alert"] button[data-id]').forEach(bttn=>bttn.addEventListener('click',updateId))
</script>

Within the PHP you really, really should use a prepared statement when dealing with user supplied data - otherwise all your hard work could be undone by one malicious user!

<?php 

    if( !empty( $_GET['id'] ) ){

        $id = $_GET['id'];
        $ip = getenv('REMOTE_ADDR');
        
        $root = realpath(str_replace('\\', '/', $_SERVER['DOCUMENT_ROOT']) );
        include ($root . '/insights/ss/onix.php'); 

        $sql='UPDATE `notifications` SET `seen`=1, `seenby`=? where `id`=?';
        
        $stmt=$mysqli->prepare($sql);
        $stmt->bind_param('ss',$ip,$id);
        $stmt->execute();
        $rows=$stmt->affected_rows;
        $stmt->close();
        
        exit( $rows ? 'Success' : 'There is some error' );
    }
?>