To Merge CSV files by Header Keys

mergcsv.php

<?php
$files_to_read = ['file1.csv', 'file2.csv', 'file3.csv'];
$all = [];
$merged_file = [];
$_row = [];
$val = '';

/**
 * Read CSV file and return data associated with it's key
 * @param $file_name
 * @return array
 */
function readCSV($file_name)
{
    $file = new SplFileObject($file_name);
    $content = [];
    $content_row = [];
    $idx2 = 0;

    $file->setFlags(SplFileObject::READ_CSV |
        SplFileObject::SKIP_EMPTY |
        SplFileObject::READ_AHEAD);

    foreach ($file as $row) {
        $new_array[] = $row;
    }

    for ($idx = 1; $idx < count($new_array); $idx++) {
        $header_columns = $new_array[0];

        foreach ($header_columns as $header_column) {
            $content[$header_column] = $new_array[$idx][$idx2];
            $idx2++;
        }
        $content_row[] = $content;
        $idx2 = 0; //reset the index for new row
    }

    return ['columns' => $header_columns, 'data' => $content_row];
}


$all_data = [];
//get combine unique columns
foreach ($files_to_read as $file_to_read) {
    $file_content = readCSV($file_to_read);
    extract($file_content);
    $all_data[] = $data;
    foreach ($columns as $column) {
        if (!(in_array($column, $all))) {
            $all[] = $column;
        }
    }
}


$merged_file[0] = implode(',', $all);
$idx_file = 0;

foreach ($all_data as $entry) {

    $f = $entry;
    foreach ($f as $item) {
        $file1 = $item;
        $_row = [];
        foreach ($all as $single) {
            $val = ($file1[$single] == null) ? "null" : $file1[$single];
            $_row[] = $val;
            $val = '';
        }
        $merged_file[] = implode(',',$_row);
        $_row = [];
    }

}

$file = new SplFileObject('merged.csv','w');
foreach ($merged_file as $line) {
    $file->fwrite($line."\n");
}