$str = "eoirgoi gwe iogjweiog erg o comoreijgi mono ierjgo.
i34jgj  iejrgi4gmon
comroejgmonogirjeoig gijergm mon iregijegi
iegj common common common erijgierg
mon";

$arr = explode("\n", $str);
foreach ($arr as $line)
{
    if (substr($line, -3) == 'mon') {
        echo "$line\n";
    }
}