PHP: Increment a variable based on change in another variable in foreach loop

Question

I have a foreach loop that prints the history of line item changes in my application. Each line item has a number, and each line item can have multiple changes.

What I want to do is display the line number alongside each change when my loop executes. $i will hold the line number.

$i = 1;
foreach($lineItem as $line) {
   echo $i; //line number
   echo $line['field_changed'];
   echo $line['change_date'];
   echo $line['change_from'];
   echo $line['change_to'];
}

The code reads from a DB table called line_item_changes with the following structure:

id  line_id  parent_id
--  -------  ---------
1   2401     521
2   2401     521
3   2401     521
4   2500     521
5   2502     521
6   2502     521  

I want to increment $i every time the value in $line['line_id'] changes. So that, when the results display, they look something like this:

Line #: 1
field: notes
date: 10/9/2018
from: test
to: test2

Line #: 1
field: quantity
date: 10/1/2018
from: 4
to: 3

Line #: 2
field: need_date
date: 10/1/2018
from: 10/24/2018
to: 10/27/2018

etc.

Answer 1

Just store the previous line_id in a variable, and update $i if the value changes. Try the following (explanation in code comments):

$i = 1;
$prev_line_id = null; // initializing the previous value

foreach($lineItem as $line) {

   // if the previous line_id exists
   if (isset($prev_line_id)) {

       // if previous value does not match with the current value
       if ($prev_line_id != $line['line_id']) {

           // increment the line number
           $i++;
       }
   }

   // set the previous value
   $prev_line_id = $line['line_id'];

   echo $i; //line number
   echo $line['field_changed'];
   echo $line['change_date'];
   echo $line['change_from'];
   echo $line['change_to'];
}

Note that you can get the changing line number directly from DB query itself. You can use window functions like Dense_Rank().

Answer 2

You have to store the last line_id of the last loop and then check it with the line_id of the next loop, if they are not equal then increment $i, when $i is incremented you set $prev_line_id to the current line_id.

$prev_line_id = $lineItem[0]['line_id'] ?? null;
$i = 0;
foreach($lineItem  as $line)
{
    if($line['line_id'] != $prev_line_id )
    {
        $i++;
        $prev_line_id = $line['line_id'];
    }

    echo $i; //line number
    echo $line['field_changed'];
    echo $line['change_date'];
    echo $line['change_from'];
    echo $line['change_to'];
}