how to add value in txt file with php - php

Question

I want to add a value (not overwrite!) to a txt file with file_put_contents
This is what i have so far:
$fileUserId = fopen("fileUserId.txt", "w") or die("Unable to open file!");
$UserIdtxt = $UserID."||";
file_put_contents("fileUserId.txt", $UserIdtxt, FILE_APPEND);
fclose($fileUserId);
$UserID is an integer, like 1, 2, 3 etc.
So when the the UserID is 1, the fileUserId.txt looks like this:
1||
When there is another user with ID 2,
the fileUserId.txt should look like this:
1||2||
But he overwrites the file so it becomes this:
2||
What i am doing wrong?

Answer 1

Remove the fopen and fclose line and you are fine. file_put_contents does this internally. And fopen("fileUserId.txt", "w") clears the file.
Note:
'w' Open for writing only; place the file pointer at the beginning of
the file and truncate the file to zero length. If the file does not
exist, attempt to create it.

Answer 2

You can as well do it differently. The commented code below illustrates how:
<?php
$txtFile = __DIR__ . "/fileUserId.txt";
$UserID = 9; //<== THIS VALUE IS FOR TESTING PURPOSES,
//<== YOU SHOULD HAVE ACCESS TO THE ORIGINAL $UserID;
//CHECK THAT THE FILE EXISTS AT ALL
if( file_exists($txtFile) ){
// GET THE CONTENTS OF THE FILE... & STORE IT AS A STRING IN A VARIABLE
$fileData = file_get_contents($txtFile);
// SPLIT THE ENTRIES BY THE DELIMITER (||)
$arrEntries = preg_split("#\|\|#", $fileData);
// ADD THE CURRENT $UserID TO THE $arrEntries ARRAY
$arrEntries[] = $UserID;
// RE-CONVERT THE ARRAY TO A STRING...
$strData = implode("||", $arrEntries);
// SAVE THE TEXT FILE BACK AGAIN...
file_put_contents($txtFile, $strData);
}else{
// IF FILE DOES NOT EXIST ALREADY, SIMPLY CREATE IT
// AND ADD THE CURRENT $UserID AS THE FIRST ENTRY...
file_put_contents($txtFile, $UserID);
}