PHP function to apply a diff

apply_diff.php

<?php

/**
 * Apply a diff to a text composed of multiple lines.
 * 
 * The diff is expected to be in unified format (https://www.gnu.org/software/diffutils/manual/html_node/Unified-Format.html)
 * but for a single file (so there are no file names and the first line starts with "@@" already).
 * 
 * The approach followed is probably not the most efficient possible, but the code is small and gets the job done.
 * 
 * @param string $contents The contents to apply the diff to.
 * @param string $diff The unified diff for the file.
 * @return string The contents with the diff applied.
 */
function apply_diff(string $contents, string $diff): string {
    $diff_lines = explode("\n", $diff);
    $added_lines = []; // line number => array of line contents
    $removed_line_numbers = [];
    foreach($diff_lines as $line) {
        if($line === '') {
            $line = ' ';
        }

        if($line[0] === '@') {
            preg_match('/-(?<original_line_number>\d+),/m', $line, $matches);
            $original_content_line_number = $matches['original_line_number'];
        }
        elseif($line[0] === '-') {
            $removed_line_numbers[] = $original_content_line_number;
            $original_content_line_number++;
        }
        elseif($line[0] === '+') {
            $added_lines[$original_content_line_number][] = substr($line, 1);
        }
        else {
            $original_content_line_number++;
        }
    }

    $new_content_lines = [];
    $current_line_number = 1;
    $original_content_lines = explode("\n", $contents);
    $max_original_content_line_number = count($original_content_lines);
    $max_line_number = max(array_merge([$max_original_content_line_number], $removed_line_numbers, array_keys($added_lines)));

    while($current_line_number <= $max_line_number) {
        $to_add = $added_lines[$current_line_number] ?? [];
        foreach($to_add as $line) {
            $new_content_lines[] = $line;
        }

        if($current_line_number <= $max_original_content_line_number && !in_array($current_line_number, $removed_line_numbers)) {
            $new_content_lines[] = $original_content_lines[$current_line_number-1];
        }

        $current_line_number++;
    }

    return implode("\n", $new_content_lines) . "\n";
}

example.php

<?php

$text = <<<MSX
Line 1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
Line 8
Line 9
Line 10
MSX;

$patch = <<<MSX
@@ -1,10 +1,15 @@
+--- Title
 Line 1
-Line 2
-Line 3
-Line 4
+Line 4 !
+aaa
+bbb
+ccc
+ddd
+eee
+fff
+ggg
 Line 5
-Line 6
-Line 7
+Line 7 ?
 Line 8
-Line 9
-Line 10
+--- Aaand...
+--- The end!
MSX;

echo apply_diff($text, $patch);

/*
Output:

--- Title
Line 1
Line 4 !
aaa
bbb
ccc
ddd
eee
fff
ggg
Line 5
Line 7 ?
Line 8
--- Aaand...
--- The end!
*/