How to replace the character in the middle of a string

How can I replace a character in the middle of a string? how can I replace a character in the middle of a string? For example, replace & nbsp; fdsfdsfdsjkww with: f ************ w for example: & nbsp; replace points with: deduct *** for example: & nbsp; replace the deduction points with: *** s. For example: & How does nb replace the characters in the middle of a string?
How to replace the character in the middle of a string?
For example, fdsfdsfdsjkww
Replace with: f *********** w
For example, the score is deducted by Jin Ximin.
Replace with: buckle *** people
For example, the score is deducted by Jin Ximin s.
Replace with: *** s
For example, tt points are deducted by Jin Ximin s.
Replace with: t ***** s

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------ Solution --------------------

 $ Str = 'TT points deducted from Kingdom s ';
Function hide_repalce ($ str, $ code = 'utf-8 '){
Mb_internal_encoding ($ code );
$ Len = mb_strlen ($ str );
Return mb_substr ($ str, 0, 1). str_repeat ('*', $ len-2). mb_substr ($ str, $ len-1, 1 );

}
Echo hide_repalce ($ str );