PHP insert string every time first character in array value changes

Question

Having the following PHP array, how can I insert a string everytime the first character of the value of 'palabra' changes?

 Array
    (
        [0] => Array
            (
                [palabra] => aaa
            )
    
        [1] => Array
            (
                [palabra] => abbb
            )
    
        [2] => Array
            (
                [palabra] => bbb
            )
    
        [3] => Array
            (
                [palabra] => ccc
            )
    
        [4] => Array
            (
                [palabra] => dddd
            )
    
        [5] => Array
            (
                [palabra] => eeee
            )
    )

I currently have something like so, but it just list

forearch ($word_array as $word) {
    echo '<li>'.$word['palabra'].'</li>';
}

The desired result is something like

<h1>Words starting with A</h1>
<li>aaa</li>
<li>abbb</li>
<h1>Words starting with B</h1>
<li>bbb</li>
<h1>Words starting with C</h1>
<li>ccc</li>

Answer 1

Track first letter on every iteration and if it changes - output h1 :

$currentLetter = '';
foreach ($word_array as $word) {
    $firstLetter = substr($word['palabra'], 0, 1);
    if ($firstLetter !== $currentLetter) {
        echo '<h1>Words starting with ' . $firstLetter . '</h1>';
        $currentLetter = $firstLetter;
    }

    echo '<li>'.$word['palabra'].'</li>';
}

As a sidenote - having h1 as item of ul will be considered as invalid markup .