I want to update a field on StartPage from a DataObject:
should remove the ID from a comma separated Field:

On StartPage: static db ……. 'MixedGalleries' => 'Varchar' ……… value is: 1,2,3

On Product- Dataobject:

function onAfterWrite(){
		parent::onAfterWrite();
			
		if($this->ShowProductImages != '1' ){
			
			$startPage = StartPage::get()->First();
			$mixedGalleriesField = $startPage->MixedGalleries;
			$fieldArray = explode(",", $mixedGalleriesField);
			if(in_array($this->ID, $fieldArray)){ // in_array  always returns Null !!!!!!  WHY??????
		
				$arr = array_diff(explode(",", $mixedGalleriesField), array($this->ID));
				$startPage->MixedGalleries = $arr;
				$startPage->write();
				$startPage->writeToStage('Stage');
				$startPage->publish("Stage", "Live");
			}
		}
	}

in_array always returns Null !
Thanks.Timo.

a hard childbirth:

function onAfterWrite(){
		parent::onAfterWrite();
			
		if($this->ShowProductImages != '1' ){
			
			$startPage = StartPage::get()->First();
			$mixedGalleriesField = $startPage->MixedGalleries;
			$fieldArray = explode(",", $mixedGalleriesField);
				
				while(($i = array_search($this->ID, $fieldArray)) !== false) {
					unset($fieldArray[$i]);
				}
				
				$string = implode(",", $fieldArray);
				$startPage->MixedGalleries = $string;
				$startPage->write();
				$startPage->writeToStage('Stage');
				$startPage->publish("Stage", "Live");
		}
	}

in_array never returns null.
http://php.net/in_array#refsect1-function.in-array-returnvalues

It will however throw a non-fatal error:

php -r "var_dump(in_array(2,'asdf'));"
PHP Warning:  in_array() expects parameter 2 to be array, string given in Command line code on line 1
NULL

Your parameter mustn't be what you think it is.

true / false
1 / 0
i ment in_array always gives me 'is not in array'
so its false ?

Here are some ideas:

	function onAfterWrite(){
		parent::onAfterWrite();
		//why do you need the quotation marks here?
		//if ShowProductImages is Boolean, you can just write if($this->MyBooleanField) {...}
		if($this->ShowProductImages != '1' ){
			//how many start pages are there?  Are you sure you are getting the right one?
			$startPage = StartPage::get()->First();
			// you may want to check you get a StartPage back before doing anything more.
			// will there always be a start page?
			$mixedGalleriesField = $startPage->MixedGalleries;
			$fieldArray = explode(",", $mixedGalleriesField);
			//I would add:
			//foreach($mixedGalleriesField as $key => $value) {$mixedGalleriesField[$key]  = intval(trim($value));}
			//for debugging purposes I would write here: print_r($mixedGalleriesField);
			// 
			if(in_array($this->ID, $fieldArray)){ // in_array  always returns Null !!!!!!  WHY??????
				// why do the exploding again?  use the variable from above and in case 
				// you are planning to change mixedGalleries field copy it to 
				// a base sort of variable that you can come back to again and again
				$arr = array_diff(explode(",", $mixedGalleriesField), array($this->ID));
				$startPage->MixedGalleries = $arr;
				$startPage->write();
				//line below is superfluous
				$startPage->writeToStage('Stage');
				$startPage->publish("Stage", "Live");
			}
		}
	}

Hi Nicolaas!

Thanks for your suggestions.
@ //I would add:
//foreach($mixedGalleriesField as $key => $value) {$mixedGalleriesField[$key] = intval(trim($value));}
//for debugging purposes I would write here: print_r($mixedGalleriesField);

I only want to remove $this->ID from the $mixedGalleriesField on StartPage.
(If ! ShowProductImages on this Product remove this ID from $mixedGalleriesField on StartPage )

StartPage :

public function getRandomGalleries() {
		$allGalleries = ArrayList::create();
		foreach(Dataobject::get('Product')->filter(array('ShowProductImages' => '1')) as $gallery) {
			$existingGallery = $gallery;
			$allGalleries->merge($existingGallery);
		}
		return $allGalleries;
	}

public function getCMSFields() {
............
$mixedGallery = $this->getRandomGalleries();
$mixedGalleryMap = $mixedGallery->map('ID', 'Title', '(Select one)', true);

if($this->ShowMixedGallery){
			$fields->addFieldToTab('Root.GalerieBilder', new ToggleCompositeField('Mixed Galerie', 'Mixed Galerie', 
			array( new CheckboxSetField( $name = "MixedGalleries", $title = "choose galleries", $source = $mixedGalleryMap, $value = ''))));
			
			//TotalGalleryImages (0-40)
			$fields->addFieldToTab('Root.GalerieBilder', new DropdownField('TotalGalleryImages', 'Anzahl Bilder', $this->getNumbers()));  
		}
...........

AND HERE COMES THE NEXT PROBLEM:

Im trying to merge all Products where ShowProductImages == 1 && checked in CheckboxSetField (on StartPage)
I came up with this but i can not get it working with ->sort('RAND()')->limit($startPage->TotalGalleryImages) :
( I need $gallery->MyImages() AND mylink' => $gallery->Link() )

Page_Controller:

public function getMixedGallery(){
		
		$startPage = StartPage::get()->First();
		$mixedGalleriesField = $startPage->MixedGalleries;
		$fieldArray = explode(",", $mixedGalleriesField);
		
		$images = ArrayList::create();
		foreach($fieldArray as $id){
			foreach(Dataobject::get('Product')->filter(array('ID' => $id, 'ShowProductImages' => '1')) as $gallery) {
				if($gallery->MyImages()->Count() > 0){
					
				$imagesForGallery = array('myimage' => $gallery->MyImages(), 'mylink' => $gallery->Link());
				$images->push($imagesForGallery);
				
				}
			}
		}
		return $images;

StartPage.ss :

<div class="gallery-content" >
					<% if $ShowGallery %>	
						<% if $ShowMixedGallery == 1 %>
							<% include Gallery gallery=$MyGallery , mixed=$ShowMixedGallery , Mixed=$getMixedGallery() %>
						<% else %>
							<% include Gallery gallery=$MyGallery , mixed=$ShowMixedGallery %>
						<% end_if %>
					<% end_if %>
				</div>

Gallery.ss :

	<% if $gallery == '1' %>
	
		<div id="gallery-holder" style=" margin: 0px -0px 0px -4px; padding: 0px 0px 0px 0px; ">
			<div id="gallery" class="masonry">
			
				<% loop Mixed %>
					<% loop myimage %>
				
				
					<div class="box masonry-brick img-responsive" >
						<a href=" $Up.mylink " class="lightbox alpha dark " rel="myscale[$Top.ID]">
							<img class="gallery_img img-responsive img-rounded-light" src="<% with $Image %>$CroppedImage(100,100).Link<% end_with %>"  alt="$Title" style=""/>
						</a>
					</div>
					
					<% end_loop %>
				<% end_loop %>
				
			</div>
		</div>
			
	<% end_if %>

Thanks.timo.