Description:
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Please add a function that is directly able to add a char or word at a numeric place in a string.

Reproduce code:
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// For example:

str_ins([inser string], [int position], [string haystack]);



// For use like this:

$longtext = 'This is a long text...';

$found = 7;

echo str_ins("_a", $found-2, $longtext);

Expected result:
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This_a is a long text...

Actual result:
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An error *lol*