PHP ftp_put fails randomly - php

Question

When moving our site to a new host we went from PHP5 (I think) to PHP7. We also added SSL to the site for the first time. Ever since moving the site a function to copy image files to an FTP has been failing randomly.
After doing some research I learned that there is no way to get an error message more detailed that "ftp_put has failed".
$dir = 'path/to/folder';
$a = scandir($dir);
$ftp_server = "ftp.server.com";
$ftp_conn = ftp_connect($ftp_server);
$ftp_username = 'myuser';
$ftp_userpass = 'mypass';
$login = ftp_login($ftp_conn, $ftp_username, $ftp_userpass);
ftp_pasv($ftp_conn, true);
foreach ($a as $value) {
if(strlen($value) > 4){
$file = $dir.$value;
$name = $value;
if (ftp_put($ftp_conn, $name, $file, FTP_BINARY)){
echo "<br><br><span style='color: green'>Successfully uploaded $file.</span><br><br>";
}
else{
echo "<br><br><span style='color: green'>Error uploading $file.</span><br><br>";
}
}
}
The output from the code above is:
Successfully uploaded ../../img/bil/AAA123/AAA123-1.jpg.
Successfully uploaded ../../img/bil/AAA123/AAA123-2.jpg.
Successfully uploaded ../../img/bil/AAA123/AAA123-3.jpg.
Error uploading ../../img/bil/AAA123/AAA123-4.jpg.
Error uploading ../../img/bil/AAA123/AAA123-5.jpg.
Successfully uploaded ../../img/bil/AAA123/AAA123-6.jpg.
Error uploading ../../img/bil/AAA123/AAA123-7.jpg.
Successfully uploaded ../../img/bil/AAA123/AAA123-8.jpg.
This output differs, running it again will successfully upload other images and fail other.
I have tried stripping down the code, removing the scandir and foreach parts and using a direct path to one image file as $file with the same result.
I have no idea what could be wrong. I suspect moving to PHP7 and possibly SSL is the problem since this all started then. Not being able to get a detailed error message why ftp_put fails leaves me completely stuck.
Is there anything I can do to find out whats wrong?
edit:
Adding error_reporting(-1) and printing out error_get_last() gives me this:
Array (
[type] => 2
[message] => ftp_put(): Type set to I
[file] => path/to/file.php
[line] => 51
)

Answer 1

Any network communication can fail.
Uploading a large number of files without zipping them up first via FTP is a good way to ensure some of your uploads will fail. If you don't want to zip them up into a single request than a good alternative is to retry failed responses, adding a delay between retries but don't make an infinite loop. Retry 3 times and if it still fails than your problem is much bigger than minor network issues or FTP server bugs and you are better off skipping the file and trying the next. Also don't forget to close any connection you open.
$dir = 'path/to/folder';
$a = scandir($dir);
foreach ($a as $value) {
if(strlen($value) > 4){
for($retry = 0; $retry < 3; $retry++) {
$ftp_server = "ftp.server.com";
$ftp_conn = ftp_connect($ftp_server);
$ftp_username = 'myuser';
$ftp_userpass = 'mypass';
$login = ftp_login($ftp_conn, $ftp_username, $ftp_userpass);
ftp_pasv($ftp_conn, true);
$file = $dir.$value;
$name = $value;
if (ftp_put($ftp_conn, $name, $file, FTP_BINARY)){
echo "<br><br><span style='color: green'>Successfully uploaded $file.</span><br><br>";
break;
} else {
if ($retry < 2) {
echo "<br><br><span style='color: green'>Error uploading $file Will retry....</span><br><br>";
sleep(2);
} else {
echo "<br><br><span style='color: green'>Error uploading $file.</span><br><br>";
}
}
ftp_close($ftp_conn);
}
}
}