php not backing up Mysql databases using exec() for mysqldump [closed] - php

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I'm using php to automate a backup process of my databases, process runs with no error and create the .sql.gz files, but all of them are of a fixed size of 350 byte, useless. I'm using exec() function to read the return value and get the output from mysqldump.
return is always 0 and I cannot print the output to see if there is an error.
This is the code I'm using:
#Process begins
$conn = new mysqli("localhost", $user, $pwd);
$result = $conn->query("SHOW DATABASES");
if ($conn->affected_rows > 0){
$conn->close();
while( $db = mysqli_fetch_row($result)){
$return=null;
$output=null;
if (($db[0]!="information_schema") && ($db[0]!="mysql")) {
$filename = "$targetdir/$db[0]-$filetime.sql";
$cmd = "mysqldump -u $user -p$pwd $db | gzip > $filename.gz";
exec($cmd, $output, $return);
if ($return==0) {
file_put_contents($logfile, serialize($output), FILE_APPEND);
} else {
file_put_contents($logfile, $return, FILE_APPEND);
file_put_contents($logfile, serialize($output), FILE_APPEND);
}
}
}
}else{
file_put_contents($logfile, "No databases found to backup", FILE_APPEND);
}
exit(0);
If anybody can help, specially to read the output I will gladly appreciate it