Chord transposition

<?php
$s_OriginalSequence = <<< END_SEQ
A
Asus7
Csus add9
Dmaj7
F#m7
G#
C#dim no3
END_SEQ;

$s_TransposedUpSequence = <<< END_SEQ
A#
A#sus7
C#sus add9
D#maj7
Gm7
A
Ddim no3
END_SEQ;

$s_TransposedDownSequence = <<< END_SEQ
G#
G#sus7
Bsus add9
C#maj7
Fm7
G
Cdim no3
END_SEQ;

/**
 * Transpose a chord sequence.
 *
 * @param $s_Sequence The chord sequence to transpose.
 * @param $b_TransposeUp Transpose up or down.
 *
 * @return The transposed sequence.
 */
function transposeChord($s_Sequence, $b_TransposeUp = True) {
    $a_Chords = array('A', 'A#', 'B', 'C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#');
    return preg_replace_callback (
        '`([A-G]#?)`',
        function($a_Matches) use($a_Chords, $b_TransposeUp) {
            // Find the position in the chords array of the found chord.
            $a_ChordPositions = array_keys($a_Chords, $a_Matches[1], True);

            // Get the new chord position.
            $i_NewChordPosition = (count($a_Chords) + $a_ChordPositions[0] + ($b_TransposeUp ? 1 : -1)) % count($a_Chords);

            // Return the new chord position.
            return $a_Chords[$i_NewChordPosition];
        },
        $s_Sequence
    );
}

$s_TestTransposedUpSequence = transposeChord($s_OriginalSequence, True);
$s_TestTransposedDownSequence = transposeChord($s_OriginalSequence, False);

echo 'Original Sequence', PHP_EOL, $s_OriginalSequence, PHP_EOL, PHP_EOL, PHP_EOL;
echo 'Transposed Up', PHP_EOL, $s_TransposedUpSequence, PHP_EOL, PHP_EOL;
echo 'vs', PHP_EOL, PHP_EOL, $s_TestTransposedUpSequence, PHP_EOL, PHP_EOL;
echo (($s_TransposedUpSequence === $s_TestTransposedUpSequence) ? 'Match' : 'Fail'), PHP_EOL, PHP_EOL, PHP_EOL;
echo 'Transposed Down', PHP_EOL, $s_TransposedDownSequence, PHP_EOL, PHP_EOL;
echo 'vs', PHP_EOL, PHP_EOL, $s_TestTransposedDownSequence, PHP_EOL, PHP_EOL;
echo (($s_TransposedDownSequence === $s_TestTransposedDownSequence) ? 'Match' : 'Fail');
?>

outputs ...

Original Sequence
A
Asus7
Csus add9
Dmaj7
F#m7
G#
C#dim no3


Transposed Up
A#
A#sus7
C#sus add9
D#maj7
Gm7
A
Ddim no3

vs

A#
A#sus7
C#sus add9
D#maj7
Gm7
A
Ddim no3

Match


Transposed Down
G#
G#sus7
Bsus add9
C#maj7
Fm7
G
Cdim no3

vs

G#
G#sus7
Bsus add9
C#maj7
Fm7
G
Cdim no3

Match