Why does this work?

Here's the code:

<?php
$data['timeline'] = array();
$txnGroup = array();

$michelle=array(
array("eventdate"=>"2016-08-26", "eventtype"=>"transaction", "runningbal"=>"21.50"),
array("eventdate"=>"2016-08-26", "eventtype"=>"transaction", "runningbal"=>"32.50"),
array("eventdate"=>"2016-08-27", "eventtype"=>"transaction", "runningbal"=>"32.50"),
array("eventdate"=>"2016-08-27", "eventtype"=>"statement", "runningbal"=>"50.50"),
);

 for ($i=0; $i < count($michelle); $i++) {
	$event = $michelle[$i];
	//var_dump($event);
	if($event['eventtype']=='transaction')
	{
		$thisDate = Date('mdy', strToTime($event['eventdate']));
		$nextDate = (isset($michelle[$i+1])&&$michelle[$i+1]['eventtype']=='transaction')?Date('mdy', strToTime($michelle[$i+1]['eventdate'])):null;
		array_push($txnGroup, $event);
		//var_dump($event);
		if ($thisDate <> $nextDate) {
			//add to data
			if (count($txnGroup)!=1) {
				echo "yes";
				array_push($data['timeline'], array(
					'eventdate'=>Date('m/d/Y', strToTime($event['eventdate'])),
					'eventtype'=>'payments',
					'runningbal'=>$event['runningbal'],
					'payments'=>$txnGroup
				));
				 

			} else {
				//this is where you left off. You've got to figure out how to 
				array_push($data['timeline'], $event);
			}
			$txnGroup = array();
		}
	} 
	else 
	{
		array_push($data['timeline'], $event); //anything that's not a transaction event type gets put into the $data['timeline'] array as a regular $event
	}
 }
 var_dump($data['timeline']);
 
 foreach($data['timeline'] as $data)
 {
	echo $data['eventtype'].'<br>';
 }
 echo "<br>";
 //var_dump($txnGroup);

 

?>

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Here's a screen shot of the results:


Perfect!

Basically, I need an array within an array when I've got a group of arrays that have the same date.

Awesome!

But I do not understand how by looking at the date of the next array, determining that the current date and the next date don't match, you can wind up with the "payments" array.

Here comes array #1: array("eventdate"=>"2016-08-26", "eventtype"=>"transaction", "runningbal"=>"21.50"),

I'm looking at the date of the next array which is 2016-08-26. They're the same, so I'm going to add this to the $txnGroup array and finish.

Here comes array #2: array("eventdate"=>"2016-08-26", "eventtype"=>"transaction", "runningbal"=>"32.50"),

I'm looking ahead and the dates are NOT the same, so I'm going to move ahead, the current $txnGroup is equal to "1," so...

Bottom line: There's a flow that results in a grouping of transactions that have the same date. It's like a digital version of "Mouse Trap," and while I'm watching it work, I get lost.

How is this working?