$chords = ['C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#', 'A', 'A#', 'B'];
// Therefore, If the original chord is E and we want to transpose +1, the resulting chord is F. If we transpose +4, the resulting chord is G#.
$index = array_search('E', $chords);
// Transpose +4 to get the index of E, found above, +4 chords
$chord = $chords[$index+4];
echo 'Chord: ' + $chord;