Can't reference MySQL by column names - php

Question

I'm struggling to reference the MySQL data simultaneously by both column name and individual row number.
The code sample below does the following;
Successfully
prints the correct data to a table when looping through the whole table, referencing columns by name OR number.
when referencing individual rows in the database the correct columns by the column number
But I can't get it to successfully reference individual rows (by number) and columns (by name) at the same time.
Apologies if this is a ridiculous question, I'm new to PHP and MySQL.
I think my description is abysmal, so maybe this will help. Of the four options available (column either by name or number, and rows either individually or looping through a group) it works like this;
Column by number, each row in a loop. Yes.
Column by name, each row in a loop. Yes.
Column by number, individual row. Yes.
Column by name, individual row. NO! (as shown in the last line of code, a is a column in my database)
Does anyone please have any advice on what I'm doing wrong?
Code:
// Check connection
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$result = mysqli_query($con,"SELECT * FROM Persons");
echo "<table border='1'>
<tr>
<th>Firstname</th>
<t
h>Lastname</th>
<th>test</th>
<th>order</th>
</tr>";
$ind = 0;
while($row = mysqli_fetch_array($result)) {
echo "<tr>";
echo "<td>" . $row['firstname'] . "</td>";
echo "<td>" . $row['lastname'] . "</td>";
echo "<td>" . ($row['age'] * 1) . "</td>";
echo "<td>" . ($row['a']) . "</td>";
echo "<td>" . $ind. "</td>";
echo "</tr>";
$ind += 1;
}
echo "</table>";
$sql="SELECT * FROM Persons ORDER BY PID";
if ($result=mysqli_query($con,$sql))
{
// Prints single entry AP
print "<br>";
$rowNumber = $id;
mysqli_data_seek($result, $rowNumber);//start row is whatever you need to be the first row
$row = mysqli_fetch_row($result);
print "<br>";
//-----------------------------------------------------
$mathString = $row[1];
$description = $row[4];
echo "math string:" . $mathString . "<br>";
echo "a is said to equal:" . $description . " - " . $row['a'];