how to make my own auto increment in php?

SELECT t1.id FROM mytable AS t1 
LEFT OUTER JOIN mytable AS t1 ON t2.id = t1.id - 1
WHERE t2.id IS NULL
ORDER BY t1.id
LIMIT 1

 SET @count = 0;
 UPDATE the_table SET id = @count:= @count + 1;
 #Then after this you do your insert operations.

/* Assume that you want to insert into a table called the_table with columns id, col1, col2, col3 a new row with values for the three cols 
$value1, $value2, $value3 respectively, using an existing 'gap' in the id numbering: */

/* Get an array of all present ids: */
$arr = []; 
$q1 = mysqli_query($con,"SELECT id from the_table");
while(list($id) = mysqli_fetch_array($q1)){
$arr[] = $id;
} 

/* Get the currently largest id in the table as $largest_id */
$q2 = mysqli_query($con,"SELECT MAX(id) from the_table");
list($largest_id) = mysqli_fetch_array($q2);

/* Loop through all integers up to $largest_id + 1 */
/* And do the insert operation just one time, once you find a number not in $arr */
/* Use the $not_yet_inserted variable to break out of the loop */
$not_yet_inserted = true;
for($j = 1; $j <= $largest_id + 1; $j += 1){
    if(!in_array($j,$arr) && $not_yet_inserted ){ 
    mysqli_query($con,"INSERT INTO the_table (id, col1, col2,col3) values ('$j','$value1','$value3,'$value3'')");
    $not_yet_inserted = false;
    }
}