$deger='0 => 1.23
1 => 3.45
3 => 6.78
13 => 12';

$dizi=array();

$deger=preg_split('/\r\n|\r|\n/',$deger);

foreach ($deger as $anahtar) {
	$anahtar=explode('=>',$anahtar);
	$dizi[trim($anahtar[0])]=trim($anahtar[1]);
}

var_dump($dizi);

array(4) { [0]=> string(4) "1.23" [1]=> string(4) "3.45" [3]=> string(4) "6.78" [13]=> string(2) "12" }