Given a DNA strand, return its RNA complement (per RNA transcription) #exercism
<?php
function toRna($nucleotides) {
$dnaToRna = [
"G" => "C",
"C" => "G",
"T" => "A",
"A" => "U"
];
return strtr(strtoupper($nucleotides), $dnaToRna);
//php strtr will replace the first parameter with its complement value in second parameter
}