A prime number is a whole number greater than 1 whose only factors are 1 and itself.

Find from 1 to 100

<?php
for($i=1; $i<=100; $i++) {
	$prime = 0;
	for($j=1; $j<=$i; $j++) {
		if($i%$j == 0) {
			$prime++;
		}
	}
	if($prime==2) {
		echo $i;echo "  ";
	}
}
?>

OUTPUT: 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

Check given number is prime or not

<?php
$no=11;
	for($i=2;$i<=$no/2;$i++)
	{
		$ans=$no%$i;
		if($ans==0)
		{
				echo $no." Number is not prime";
				goto end;
		}	
				
	
	}
		echo $no." Number is prime";
		end:
?>

OUTPUT: 11 Number is prime