In Drupal 7, there was drupal_get_title()and drupal_set_title(). They're history.

I want to alter the page title "Create X" to "Add X" for all X, and I think hook_preprocess_page_title() in my .theme-file is the right place to do it. (If you disagree, please say so.)

I.e. I want it to be 'Add X' for any 'X' when it originally was 'Create X'. For example: 'Create article' should become 'Add article'. And if the title is 'Bar Article', I should be left as is.

Basically, I want to sniff the title string and if it starts with 'Create ', alter it to 'Add '. This is so far I've gotten, and this is obviously incomplete:

/**
 * Implements hook_preprocess_page_title().
 */
function guidedtheme_preprocess_page_title(&$variables) {
  $request     = \Drupal::request();
  $route_match = \Drupal::routeMatch();
  $title       = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());
}

I know that I can change the title to some translatable string by doing this:

$variables['title'] = t('Some string');

When I inspect $title, which I know have learnt is a "markup object", I can see that the attribute #translatedMarkup: is "Create @name".

However, doing this:

$variables['title'] = t('Add @name');

Changes the title of the add article form from "Create article" to "Add @name".

I think my questions boils down to the following:

1. $title is not a string. The string is somewhere in there, but how do I extract it to see what it starts with?
2. How do I actually change it in the context of a preprocess hook. I.e. what is equivalent of drupal_set_title() in this context?

I don't want to use the String Overrides module for this for several reasons, mostly because this theme should work across translations without requiring extra configuration when deployed.

In Drupal 7, there was drupal_get_title()and drupal_set_title(). They're history.

I want to alter the page title "Create X" to "Add X" for all X, and I think hook_preprocess_page_title() in my .theme-file is the right place to do it. (If you disagree, please say so.)

I.e. I want it to be 'Add X' for any 'X' when it originally was 'Create X'. For example: 'Create article' should become 'Add article'. And if the title is 'Bar Article', I should be left as is.

Basically, I want to sniff the title string and if it starts with 'Create ', alter it to 'Add '. This is so far I've gotten, and this is obviously incomplete:

/**
 * Implements hook_preprocess_page_title().
 */

First attempt:

function mytheme_preprocess_page_title(&$variables) {
  $request     = \Drupal::request();
  $route_match = \Drupal::routeMatch();
  $title       = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());
}

Second attempt, based upon suggestion by 4k4:

function mytheme_preprocess_page_title(&$variables) {
  $title = $variables['title'];
  if ($title instanceof TranslatableMarkup) {
    $title = $title->getUntranslatedString();
    $title = str_replace('Create', 'Add', $title);
    $variables['title'] = t($title);
  }
  else {
    $variables['title'] = Markup::create(str_replace('Create', 'Add', $title));
  }  
}

The titles I want to change come from Drupal core and they are translatable, so its the instanceof TranslatableMarkup branch that gets executed, but I agree with having a fallback in case they're not, is a good idea.

However, while a good start, this is not a complete solution. It changes the rendered title of the add article form from "Create article" to "Add @name".

I've upvoted the answer by 4k4, as it goes much further towards a solution than my first attempt, but it is still not complete.

I don't want to use the String Overrides module for this for several reasons, mostly because this theme should work across translations without requiring extra configuration when deployed.

In Drupal 7, there was drupal_get_title()and drupal_set_title(). They're history.

I want to alter the page title "Create X" to "Add X" for all X, and I think hook_preprocess_page_title() in my .theme-file is the right place to do it. (If you disagree, please say so.)

Basically, I want to sniff the title string and if it starts with 'Create ', alter it to 'Add '. This is so far I've gotten, and this is obviously incomplete:

/**
 * Implements hook_preprocess_page_title().
 */
function guidedtheme_preprocess_page_title(&$variables) {
  $request     = \Drupal::request();
  $route_match = \Drupal::routeMatch();
  $title       = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());
}

Problems:

1. $title is not a string. The string is somewhere in there, but how do I extract it to see what it starts with?
2. How do I actually change it in the context of a preprocess hook. I.e. what is equivalent of drupal_set_title() in this context?

I don't want to use the String Overrides module for this for several reasons, mostly because this theme should work across translations without requiring extra configuration when deployed.

In Drupal 7, there was drupal_get_title()and drupal_set_title(). They're history.

I want to alter the page title "Create X" to "Add X" for all X, and I think hook_preprocess_page_title() in my .theme-file is the right place to do it. (If you disagree, please say so.)

I.e. I want it to be 'Add X' for any 'X' when it originally was 'Create X'. For example: 'Create article' should become 'Add article'. And if the title is 'Bar Article', I should be left as is.

Basically, I want to sniff the title string and if it starts with 'Create ', alter it to 'Add '. This is so far I've gotten, and this is obviously incomplete:

/**
 * Implements hook_preprocess_page_title().
 */
function guidedtheme_preprocess_page_title(&$variables) {
  $request     = \Drupal::request();
  $route_match = \Drupal::routeMatch();
  $title       = \Drupal::service('title_resolver')->getTitle($request, $route_match->getRouteObject());
}

I know that I can change the title to some translatable string by doing this:

$variables['title'] = t('Some string');

When I inspect $title, which I know have learnt is a "markup object", I can see that the attribute #translatedMarkup: is "Create @name".

However, doing this:

$variables['title'] = t('Add @name');

Changes the title of the add article form from "Create article" to "Add @name".

I think my questions boils down to the following:

1. $title is not a string. The string is somewhere in there, but how do I extract it to see what it starts with?
2. How do I actually change it in the context of a preprocess hook. I.e. what is equivalent of drupal_set_title() in this context?

I don't want to use the String Overrides module for this for several reasons, mostly because this theme should work across translations without requiring extra configuration when deployed.