Use "\n" within the cell value (wrapped with ") and use "\r\n" as the end of record tag. If If the cell input contains multiple lines, must be surrounded by ".

$fh = fopen('test1.csv', 'w+');
fwrite($fh, "sep=\t" . "\r\n");
fwrite($fh, 'A' ."\t" . 'B' . "\t" . 'C' . "\r\n");
fwrite($fh, 'D' ."\t" . "\"E\nF\nG\"" . "\t" . 'H' . "\r\n");
fclose($fh);

$varA = 'A';
$varB = 'B';
$varC = 'C';
$varD = 'D';
$varE = 'E';
$varF = 'F';
$varG = 'G';
$varH = 'H';
$fh = fopen('test2.csv', 'w+');
fwrite($fh, "sep=\t"."\r\n");
fwrite($fh, $varA . "\t" . $varB . "\t" . $varC . "\r\n");
fwrite($fh, $varD . "\t" . "\"$varE\n$varF\n$varG\"" . "\t" . $varH . "\r\n");
fclose($fh);

or (use fputcsv())

$fh = fopen('test3.csv', 'w+');
fwrite($fh, "sep=\t" . "\r\n");
fputcsv($fh, array('A', 'B', 'C'), "\t");
fputcsv($fh, array('D', "E\nF\nG", 'H'), "\t");
fclose($fh);

fputcsv() and variables)

$varA = 'A';
$varB = 'B';
$varC = 'C';
$varD = 'D';
$varE = 'E';
$varF = 'F';
$varG = 'G';
$varH = 'H';
$fh = fopen('test4.csv', 'w+');
fwrite($fh, "sep=\t" . "\r\n");
fputcsv($fh, array($varA, $varB, $varC), "\t");
fputcsv($fh, array($varD, "$varE\n$varF\n$varG", $varH), "\t");
fclose($fh);

Try this

"\n"

Within double quotes