Populate dropdown from database

Question

If that testing_db is your WordPress database, then you should use the WordPress wpdb API/class to interact with the data in your custom table (wp_tmadm_dojangs).

So if this is how you do it in “regular PHP”, i.e. using native PHP functions:

$sql = mysqli_query($conn, "SELECT dojangName FROM wp_tmadm_dojangs");

while ($row = $sql->fetch_assoc()){
    $dojang = $row['dojangName'];
    echo "<option value="$dojang">$dojang</option>";
}

Then in WordPress, this is one way of how you can do it:

$results = $wpdb->get_results( "SELECT dojangName FROM {$wpdb->prefix}tmadm_dojangs", ARRAY_A );
var_dump( $results ); // for debugging

foreach ( $results as $row ) {
    $dojang = $row['dojangName'];
    echo "<option value="$dojang">$dojang</option>";
}

Please check the $wpdb->get_results() (or wpdb::get_results()) reference for details on the function’s syntax, parameters, etc, but what I can tell you from the above example are:

$wpdb->get_results() returns an array of rows from the queried table and by default, each row is a PHP object with each column as a property in the object, e.g. $row->dojangName.

However, in the code, you could see I used the ARRAY_A constant which gives us each row as an associative array with the table column as the array key, e.g. $row['dojangName'].

But regardless it’s an object or associative array, the value is of course the (raw) value in the corresponding database column.
If you’re retrieving a single column/field from the database table, then you can just use $wpdb->get_col() which returns the column values in an indexed array, i.e. array( 0 => 'value', 1 => 'value2', 2 => 'value3', ... ).
I presumed your WordPress database (table) prefix is wp_, and instead of hard-coding the prefix, you should use $wpdb->prefix to get the prefix.

So using $wpdb->get_col(), the example I gave would be:

$results = $wpdb->get_col( "SELECT dojangName FROM {$wpdb->prefix}tmadm_dojangs" );
var_dump( $results ); // for debugging

foreach ( $results as $dojang ) {
    echo "<option value="$dojang">$dojang</option>";
}

Btw, please make sure you escape the database values prior to presenting them to the user. E.g. echo '<option value="' . esc_attr( $dojang ) . '">' . esc_html( $dojang ) . '</option>';

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