HTML table in php foreach loop where the cell data is only populated if an element is equal to the table column head name

You can use a dictionary to store all the matches based on the rounds:

<?php

/*#########SAMPLE DATA########*/

$matches = <<<MATCHES
[{"match":{"id":246811883,"tournament_id":10128202,"state":"complete","player1_id":152239163,"player2_id":152239166,"player1_prereq_match_id":null,"player2_prereq_match_id":null,"player1_is_prereq_match_loser":false,"player2_is_prereq_match_loser":false,"winner_id":152239166,"loser_id":152239163,"started_at":"2021-08-07T01: 59: 51.967+02: 00","created_at":"2021-08-07T01: 59: 51.692+02: 00","updated_at":"2021-08-07T02: 22: 29.417+02: 00","identifier":"A","has_attachment":false,"round":1,"player1_votes":1,"player2_votes":null,"group_id":null,"attachment_count":null,"scheduled_time":null,"location":null,"underway_at":"2021-08-07T02: 05: 31.834+02: 00","optional":false,"rushb_id":null,"completed_at":"2021-08-07T02: 22: 29.455+02: 00","suggested_play_order":null,"forfeited":null,"open_graph_image_file_name":null,"open_graph_image_content_type":null,"open_graph_image_file_size":null,"prerequisite_match_ids_csv":"","scores_csv":"0-2"}},{"match":{"id":246811884,"tournament_id":10128202,"state":"complete","player1_id":152239594,"player2_id":152239168,"player1_prereq_match_id":null,"player2_prereq_match_id":null,"player1_is_prereq_match_loser":false,"player2_is_prereq_match_loser":false,"winner_id":152239594,"loser_id":152239168,"started_at":"2021-08-07T01: 59: 51.984+02: 00","created_at":"2021-08-07T01: 59: 51.698+02: 00","updated_at":"2021-08-07T02: 32: 30.655+02: 00","identifier":"B","has_attachment":false,"round":1,"player1_votes":2,"player2_votes":null,"group_id":null,"attachment_count":null,"scheduled_time":null,"location":null,"underway_at":"2021-08-07T02: 05: 23.667+02: 00","optional":false,"rushb_id":null,"completed_at":"2021-08-07T02: 32: 30.690+02: 00","suggested_play_order":null,"forfeited":null,"open_graph_image_file_name":null,"open_graph_image_content_type":null,"open_graph_image_file_size":null,"prerequisite_match_ids_csv":"","scores_csv":"2-1"}}]
MATCHES;

$round = <<<ROUND
[-7, -6, -5, -4, -3, -2, -1, 1, 2, 3, 4, 5, 6]
ROUND;

$matches = json_decode($matches, true);
$round = json_decode($round);


/*###########################*/

//class to define data when we found a match
class MatchInfo {
    public $firstCharacter;
    public $lastCharacter;
    public $player1Votes;
    public $player1Name;
    public $player1Misc;
    public $player2Name;
    public $player2Misc;
}

$tableHeaders = null;
$matchInfoData = [];
//$participants = [...]

if (!empty($round)) { //if we have round
    //get the html for table headers
    $tableHeaders = array_map(function($colHead){
        return "<th>Round: {$colHead}</th>";
    }, $round);
    $tableHeaders = implode("", $tableHeaders);

    //remove item in matches
    while ($match = array_pop($matches)) {
        $targetRound = $match['match']['round'];
        if (!in_array($targetRound, $round)) continue; //skip item if match round is not in our column

        if (!array_key_exists($targetRound, $matchInfoData)) { //if we never look at this round before
            $matchInfoData[$targetRound] = []; //initialize array to hold other match based on the round
        }


        $id1 = $match['match']['player1_id'];
        $id2 = $match['match']['player2_id'];
        //new up and save off the data
        $matchInfo = new MatchInfo();
        $matchInfo->firstCharacter = $match['match']['scores_csv'][0];
        $matchInfo->lastCharacter = $match['match']['scores_csv'][-1];
        $matchInfo->player1Votes = $match['match']['player1_votes'];
        $matchInfo->player1Name = "Player1 Name"; //$participants[$id1]["name"];
        $matchInfo->player1Misc = "Player1 Misc"; //$participants[$id1]["misc"];
        $matchInfo->player2Name = "Player2 Name"; //$participants[$id2]["name"];
        $matchInfo->player2Misc = "Player2 Misc"; // $participants[$id2]["misc"];

        $matchInfoData[$targetRound][] = $matchInfo; //add data to the round...array_push
    }

}

//if no matches found exit
if (empty($matchInfoData)) {
    echo "No match info";
    exit;
}

$tableData = '';
//now we loop through all the array to be sure its the same order as the ths
foreach ($round as $td) {
    $columnHtml = '';
    if (array_key_exists($td, $matchInfoData)) { //if what we stored contains the targeted column
        //create a td html
        $dataHtmlArray = array_map(function(MatchInfo $curItem){
            return <<<ITEMDATA
    <div class="match-info">
        Table: {$curItem->player1Votes} <br/>
        {$curItem->player1Name} {$curItem->player1Misc} {$curItem->firstCharacter} <br/>
        {$curItem->player2Name} {$curItem->player2Misc} {$curItem->lastCharacter } <br/>
    </div>
ITEMDATA;

        },$matchInfoData[$td]);

        $dataHtml = implode("<br/>", $dataHtmlArray);
        $columnHtml = "<td><div class='match-container'> {$dataHtml} </div></td>";

    }else { //otherwise it needs a blank td to go to next column
        $columnHtml = "<td></td>";
    }
    $tableData .= $columnHtml;
}

?>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.0/dist/css/bootstrap.min.css" integrity="sha384-KyZXEAg3QhqLMpG8r+8fhAXLRk2vvoC2f3B09zVXn8CA5QIVfZOJ3BCsw2P0p/We" crossorigin="anonymous">

<table class="table table-bordered">
    <thead>
    <tr><?php echo $tableHeaders; ?></tr>
    </thead>
    <tbody>
    <tr>
        <?php echo $tableData; ?>
    </tr>

    </tbody>
</table>

<style>
    .match-info {
        border: 1px solid #dadde5;
        padding: 24px;
        border-radius: 4px;
    }
</style>

A few comments up front:

Your $round array has one element yet you have a foreach statement iterating "all" of the elements ...

foreach ($round as $colhead): array_map('htmlentities', $colhead);

... which doesn't make much sense semantically because a table should not have multiple headers. Then you invoke array_map on the $colhead list but the results are not saved and used later on. Moreover, as these values are integers, there is no reason to even apply the htmlentities function to these values. Later on, when you are putting out entities such as names, is where this function could prove useful.

The following code builds a map between a round number and the index into the $matches array where that round number can be found. Then the processing becomes greatly simplified. Just for demo purposes:

1. I am only outputting for each match the player1_id value so that the details of processing do not get lost and for simplicity sake I have set this value to be the same as the round number.
2. If a matching round is not found, the table column is just filled with a <br>.
3. Also for simplicity sake I have defined the $round array to contain one element, a list with values [-1, -3, -5] and the $matches array will only have data for two table rows. The first row for round numbers -1 and -5 (-3 will be missing) and the second row will have all three round numbers.

<?php
$round = [[-1, -3, -5]];

$match0 = [];
$match0[] = Array('match' => Array('round' => 2, 'player1_id' => 2));
$match0[] = Array('match' => Array('round' => 1, 'player1_id' => 1));
$match0[] = Array('match' => Array('round' => 3, 'player1_id' => 3));
$match0[] = Array('match' => Array('round' => -1, 'player1_id' => -1));
$match0[] = Array('match' => Array('round' => -4, 'player1_id' => -4));
$match0[] = Array('match' => Array('round' => -5, 'player1_id' => -5));
$match0[] = Array('match' => Array('round' => -6, 'player1_id' => -6));

$match1 = [];
$match1[] = Array('match' => Array('round' => -7, 'player1_id' => -7));
$match1[] = Array('match' => Array('round' => -1, 'player1_id' => -1));
$match1[] = Array('match' => Array('round' => -2, 'player1_id' => -2));
$match1[] = Array('match' => Array('round' => -3, 'player1_id' => -3));
$match1[] = Array('match' => Array('round' => -8, 'player1_id' => -8));
$match1[] = Array('match' => Array('round' => 9, 'player1_id' => 9));
$match1[] = Array('match' => Array('round' => -5, 'player1_id' => -5));

$matches = [$match0, $match1];


if (count($round) > 0):
    $round0 = $round[0];
?>
<table>
  <thead>
    <tr>
<?php
    foreach($round0 as $hdr):
?>
      <th><?= $hdr ?></th>
<?php
    endforeach;
?>
    </tr>
  </thead>
  <tbody>
<?php
    foreach ($matches as $match):
        // create mapping between round number and index into $matches:
        $index_map = [];
        $l = count($match);
        for ($i = 0; $i < $l; $i++):
            $matchRound = $match[$i]['match']['round'];
            $index_map[$matchRound] = $i;
        endfor;
?>
    <tr>
<?php
        foreach($round0 as $matchRound):
            if (!array_key_exists($matchRound, $index_map)):
?>
      <th><br></th>
<?php
            else:
                $theMatch = $match[$index_map[$matchRound]]['match']
?>
      <th><?= $theMatch['player1_id'] ?></th>
<?php
            endif;
        endforeach;
    endforeach;
?>
    </tr>
  </tbody>
</table>
<?php
endif;
?>

Outputs:

<table>
  <thead>
    <tr>
      <th>-1</th>
      <th>-3</th>
      <th>-5</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <th>-1</th>
      <th><br></th>
      <th>-5</th>
    <tr>
      <th>-1</th>
      <th>-3</th>
      <th>-5</th>
    </tr>
  </tbody>
</table>

You already have all your header columns in $colhead.

So, you can iterate the columns, like this:

foreach ($matches as $match) {
    $colindex = 0;
    for ($colindex = 0; $colindex < count($colhead), $colindex++) {
        if ($match['match']['round'] === $colhead[$colindex]) {
            //your cell template here
        }
    }
}