Prime numbers in PHP checking and generating

Correctly test whether a number is prime

A prime number is an integer greater than 1 with exactly two positive factors: 1 and itself. Values below 2 are therefore not prime. Testing divisors only up to the square root is sufficient.

<?php
function isPrime(int \): bool
{
    if (\ < 2) {
        return false;
    }

    for (\ = 2; \ * \ <= \; \++) {
        if (\ % \ === 0) {
            return false;
        }
    }

    return true;
}

echo isPrime(18) ? 'Prime number' : 'Not a prime number';
?>

Prime numbers are having only two factors, one is 1 and other one is the number itself.
These are the first 5 prime numbers 2, 3, 5, 7, 11
Prime numbers are divisible by exactly two natural numbers, 1 and the number itself.

Checking and listing all prime numbers in PHP by using loop and form to take user inputs

Checking numbers

Any number we can check if it is Prime number or not .

\=18;
\=0;
for(\=2;\<=(\/2); \++){
	if(\%\ == 0 ){
	\=1;
	break;
	}
}

if(\==0){
	echo  \"$n1 is a prime number \" ;
}else{
	echo  \"$n1 is NOT a prime number \" ;
}

Output

18 is NOT a prime number

Inside the if condition check we userd break; to come out of the for loop. This is a better way of coding as there is no need to check further if any one of the reminder of division ( or modulus ) became 0.

Sticky form to take user input

We will ask user to enter any number in a sticky form and then display the number is prime number or not.
2 is a prime number

\2=\Array['n1']; // change this number 
echo \"<form method=POST action=''>
<input type=text name=n1 value='$n1'>
<input type=Submit value=submit></form>\";
$flag=0;
for($i=2;$i<=($n1/2); $i++){
	if($n1%$i == 0 ){
	$flag=1;
	break;
	}
}

if($flag==0){
	echo  \"$n1 is a prime number \" ;
}else{
	echo  \"$n1 is NOT a prime number \" ;
}

Listing all prime numbers

We will loop through a range of numbers and check and list of all the prime numbers. ( All prime numbers less than 100 )

for(\=2;\<=100;\++){
\2=\;	
\0=0;
for(\2=2;\2<=(\2/2); \2++){
	if(\2%\2 == 0 ){
	\0=1;
	break;
	}
}

if(\0==0){
	echo  \"$n1 , \" ;
}	
}

Output is here

2 , 3 , 5 , 7 , 11 , 13 , 17 , 19 , 23 , 29 , 31 , 37 , 41 , 43
 , 47 , 53 , 59 , 61 , 67 , 71 , 73 , 79 , 83 , 89 , 97 ,

Introduction to PHP Factorial of a number Sum of Digits of a number
Armstrong number Strong number
Basic Codes Check the string or number is Palindrome or not in PHP

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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Prime Numbers in PHP

Correctly test whether a number is prime

Checking numbers

Sticky form to take user input

Listing all prime numbers

Subhendu Mohapatra