About PHP static variables

Don’t PHP’s static variables only store one copy in memory? I tried the following code today and I have some questions

function test(){
    static $sum = 0;
    static $sum = 20;
    for ($i=0; $i < 100; $i++) { 
        $sum = $sum + $i; 
    }
    echo  $sum;
}
echo "
";
    test();//4970
echo "
";
    test();//9920
echo "
";
    test();//14870
    
    
Since there is only one copy in the memory, calling it again is similar to a direct reference, so why was $num assigned to 20 for the first time? 
 Shouldn’t the result of the first run be 4950?

                            
                        


                                                                                                                        
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Don’t PHP’s static variables only store one copy in memory? I tried the following code today and I have some questions


function test(){
    static $sum = 0;
    static $sum = 20;
    for ($i=0; $i < 100; $i++) { 
        $sum = $sum + $i; 
    }
    echo  $sum;
}
echo "
";
    test();//4970
echo "
";
    test();//9920
echo "
";
    test();//14870
    
    
Since there is only one copy in the memory, calling it again is similar to a direct reference, so why was $num assigned to 20 for the first time? 
 Shouldn’t the result of the first run be 4950?

                            
                        
            

                                    
$i =1;$i<101
…


//Result: 4950;

                            
            

                                    
It can be understood that the line with the value 20 overwrites the line with the value 0 above. Because the variable names are the same, it is 20 when initialized.