Замена букв, на указанные

DNA

<!--     $this->assertEquals("TTTT", DNA_strand("AAAA"));
$this->assertEquals("AAAA", DNA_strand("TTTT"));
$this->assertEquals("TAACG", DNA_strand("ATTGC"));
$this->assertEquals("ATTGC", DNA_strand("TAACG"));
$this->assertEquals("CATA", DNA_strand("GTAT"));
$this->assertEquals("GTAT", DNA_strand("CATA")); -->

function DNA_strand($dna) {
  $from= "ATCG";
  $to = "TAGC";
  $DNA = strtr($dna, $from, $to);
  echo $DNA;
}