How can I divide data separated by commas on different lines

How can I divide data separated by commas on different lines

I have this script that extracts a .csv file from the database that holds
data for different locals that a user has logged into. The .csv files come
like this:
"id_user";"id_local"
"1";""
"2";"2,3,4"
"3";""
"5";"2,5"
"10";""
"13";"2"
"14";"5"
"15";"2"
"16";"1"
"20";"2"
"21";""
As you can se, it get one register per user But, to manipulate it
properly, we need it like this:
"id_user";"id_local"
"2";"2"
"2";"3
"2";"4"
"5";"2"
"5";"5"
"13";"2"
"14";"5"
"15";"2"
"16";"1"
"20";"2"
So, I need to create a function that deletes users with no local and
splits different locals of the same user in different registers. Does
anyone knows how can I do it?
Here is the code I have so far but I'm not sure if I'm on the right way:
function fix_local_secundario(){
$filename = "local_secundario.csv";
$file_locais = file_get_contents($filename);
$locais = explode("\n", $file_locais);
// $pattern = "/,/";
// $replacement = "\"\n;\"";
while ($line = current($locais)) {
$line = str_getcsv($line, ';', '"','\n');
// $line = preg_replace($pattern, $replacement, $line);
var_dump($line);
echo "\n";
next($locais);
}
}