# PHP Program to Check Prime Number

A Prime number is a natural number greater than 1 and divisible by 1 and itself only, for example: 2, 3, 5, 7, etc.

### Method 1: Using conditional statements

In the example below, the number called MyNum is checked for prime number by dividing it with all natural numbers starting from 2 to N - 1.

```<?php
\$MyNum = 17;
\$n = 0;

for(\$i = 2; \$i < \$MyNum; \$i++) {
if(\$MyNum % \$i == 0){
\$n++;
break;
}
}

if (\$n == 0){
echo \$MyNum." is a prime number.";
} else {
echo \$MyNum." is not a prime number.";
}
?>
```

The above code will give the following output:

```17 is a prime number.
```

### Method 2: Using function

In the example below, a function called primenumber() is created which takes a number as argument and checks it for prime number by dividing it with all natural numbers starting from 2 to N/2.

```<?php
\$n = 0;

for(\$i = 2; \$i < (\$MyNum/2+1); \$i++) {
if(\$MyNum % \$i == 0){
\$n++;
break;
}
}

if (\$n == 0){
echo \$MyNum." is a prime number.";
} else {
echo \$MyNum." is not a prime number.";
}
}

?>
```

The above code will give the following output:

```21 is not a prime number.
```

### Method 3: Optimized Code

• Instead of checking the divisibility of given number from 2 to N/2, it is checked till square root of N. For a factor larger than square root of N, there must the a smaller factor which is already checked in the range of 2 to square root of N.
• Except from 2 and 3, every prime number can be represented into 6k ± 1.
```<?php
\$n = 0;
if (\$MyNum == 2 || \$MyNum == 3) {
echo \$MyNum." is a prime number.";
} elseif (\$MyNum % 6 == 1 || \$MyNum % 6 == 5) {
for(\$i = 2; \$i*\$i <= \$MyNum; \$i++) {
if(\$MyNum % \$i == 0){
\$n++;
break;
}
}

if (\$n == 0){
echo \$MyNum." is a prime number.";
} else {
echo \$MyNum." is not a prime number.";
}
} else {
echo \$MyNum." is not a prime number.";
}
}

```21 is not a prime number.