Working example of the pecl ssh2 extension copying files to remote.

pecl_ssh2.php

<?php declare(strict_types=1);
// Tested on
// PHP 7.4.14 (cli) (built: Jan  5 2021 15:12:29) ( ZTS Visual C++ 2017 x64 )
// Windows 10 Pro
// pecl ssh2 v1.2

// Remote server
// Gentoo GNU/Linux Base System release 2.7

error_reporting(E_ALL);
ini_set('display_errors', '1');

file_put_contents(__DIR__.'/sftp.test', 'worked!');

$host = 'your.remote.ip.address';
$port = 22;

$connection = ssh2_connect($host, $port); // let libssh2 figure it out.

if(!is_resource($connection)) {
    throw new Exception('Connection failed.');
}

// Without the __DIR__ php will just try to get 'id_rsa' and will fail.
// if your keys are elsewhere __DIR__ should be changed to C:\path\to\your\files
$privateKey = __DIR__.'\your_private_key_file'; // id_rsa
$publicKey = __DIR__.'\your_public_key_file.pub'; // id_rsa.pub
$username = 'your_username';
$passphrase = ''; // set this to passphrase if you have one.

if(false === ssh2_auth_pubkey_file($connection, $username, $publicKey, $privateKey, $passphrase)) {
    ssh2_disconnect($connection);
    throw new Exception('Authentication failed.');
}

$sftp = ssh2_sftp($connection);
$streamUrl = sprintf("ssh2.sftp://%d", (int)$sftp);

$path = sprintf('%s/path/to/the/directory', $streamUrl);
$file = sprintf('%s/sftp.test', $path);
$localFile = __DIR__ . '/sftp.test';

if(!file_exists($file)) {
    if(true === copy($localFile, $file)) {
        print 'Successfully copied ' . $localFile . PHP_EOL;
    }
} else {
    print 'File exists' . PHP_EOL;
}

// using ssh2_scp_send, if all you need to do is upload a file.
// There doesn't seem to be another way to check if the file exists
// using just the ssh2 resource without using the ssh2.sftp stream wrapper.
// If you want to check if the file exists use ssh2_sftp_stat() it will
// return false if the file does not exist.
$file2 = '/path/to/the/file/you/want/to/write/sftp.test2';

$res = ssh2_scp_send($connection, $localFile, $file2);
if($res) {
    print 'Sent file ' . $file . PHP_EOL;
} else {
    print 'There was an error copying file to remote.' . PHP_EOL;
}

// List contents of remote directory
$i = new \RecursiveDirectoryIterator($path, \RecursiveDirectoryIterator::SKIP_DOTS);

while($i->valid()) {
    print $i->getPathname();
    if($i->isDir()) {
        print $i->getPath() . PHP_EOL;
    } else {
        print $i->getFilename() . PHP_EOL;
    }
    $i->next();
}


// shutdown
ssh2_disconnect($sft); // If you don't unset $sftp ssh2_disconnect will throw a segfault.
ssh2_disconnect($connection);

No worries!
Before php 5.6 It was standard to use them. Since then they have suggested not using them unless you have mixed content.

I Hope this helps!