sample.php

<?php
// file name to put json
$fileName = '/path/to/data.json';
/**
 * This just sample stmt
 * @var \PDO $pdo
 * @var \PDOStatement $stmt
 */
$stmt = $pdo
    ->query("SELECT * FROM XXXXXXXX");
// do execute
$stmt->execute();

// create socket
$fp = fopen($fileName, 'w+');
// write open square bracket
$dataWritten = fwrite($fp, '[');
// doing db fetch
$isFirst = true;
while($row = $stmt->fetch()) {
    // add commas cause this is array
    $data = sprintf(
        '%s%s',
        $isFirst ? '' : ',', // if first do not prepend comma otherwise prepend
        json_encode($row, JSON_UNESCAPED_SLASHES) // encode result
    );
    $isFirst = false;
    // append data per query
    $dataWritten += fwrite(
        $fp,
        $data
    );
}

// append square bracket as array  close increment
$dataWritten += fwrite($fp, ']');
fclose($fp);
// close database cursor
$stmt->closeCursor();
unset($stmt);

printf("Written %d total bytes data on %s.\n", $dataWritten, $fileName);