Help me write a natural number summation function, thanks! Solution

Help me write a natural number summation function, thanks !!! For example, if 3 is input to a function, the result after calculation is 6 (1 + 2 + 3 ); if 6 is input to a function, the result after function calculation is 21 (1 + 2 + 2 + 4 + 5 + 6 )., funct helps you write a natural number summation function, thanks !!!
For example, if 3 is input to a function, the result after calculation is 6 (1 + 2 + 3 ); if 6 is input to a function, the result calculated by the function is 21 (1 + 2 + 2 + 4 + 5 + 6 ).
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Function foo ($ n ){
$ Ar = range (1, $ n );
Return array_sum ($ ar );
}
Echo foo (6 );

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Function foo ($ n ){
Return $ n * ($ n + 1)/2;
}