php explode between two delimiter and delimiter not lost

Instead of splitting, you could also match the parts that you want by matching optional horizontal whitespace chars, and then capture in group 1 as least as possible chars followed by matching [...]

For the matches get the group 1 value using $matches[1]

\h*(.*?\[[^][]*])

Regex demo | Php demo

Example code

$s = "i want to show you my youtube channel : [youtube id=12341234] and my instagram account : [instagram id=myingaccount213]";
preg_match_all("~\h*(.*?\[[^][]*])~", $s, $matches);
print_r($matches[1]);

Output

Array
(
    [0] => i want to show you my youtube channel : [youtube id=12341234]
    [1] => and my instagram account : [instagram id=myingaccount213]
)

Another option is making the pattern a bit more specific for youtube or instagram:

\h*(.*?\[(?:youtube|instagram)\h+id=[^][\s]+])

Regex demo

If the string will only contain one

$newStr = substr($str, strpos($str, '[') -1, strlen($str) - strpos($str, ']');

or

preg_match('/\[.+\]/', $str, $matches, PREG_OFFSET_CAPTURE);

Please try:

$str = explode('|', str_replace('] ', ']|', $str));