How to determine whether a number is prime in php?

Prime numbers are also called prime numbers. A natural number greater than 1 that cannot be divided by other natural numbers except 1 and itself is called a prime number; otherwise it is called a composite number. (Note: 1 is not a prime number.) So how does PHP determine whether a number is prime? The following article will introduce it to you.

Let’s introduce what are the three ways to determine prime numbers in php?

Method 1:

Basic method, - counting method.

$num = 7;$n = 0; //用于记录能被整除的个数 -- 计数
for($i = 1;$i <= $num; ++$i){    
    if($num % $i == 0){        
       $n++;
    }
}
if($n == 2){    
   echo "$num 是素数";
}else{    
   echo "$num 不是素数";
}

Method 2:

That is, when a number is equal to the product of two numbers, one of the numbers must be less than half of the number. Use break; as long as one of the numbers can be divided, the loop will end immediately. This reduces the number of loops and speeds up the process.

$num = 5;$flag = true;
for($i = 2;$i <= $num/2;++$i)
{    if($num % $i == 0)
    {        $flag = false;        break;
    }
}if($flag)
{    echo "$num 是素数";
}else{    echo "$num 不是素数";
}

Method 3:

Same as above, when the product of two numbers is equal to one number, then one of the numbers must be less than the square root of the number.

$num = 4;for($i = 2;$i<$num;++$i){    
     if($num % $i == 0){        
         echo "$num 不是素数";        
         break;
    }    
    if($i >= sqrt($num)){        
       echo "$num 是素数";        
       break;
    }
}

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