<?php
/* The Sieve of Eratosthenes implemented in PHP
 * Robin Broad
 * February 2019
 * Part of a project to demonstrate an ability to
 * implement an algorithm in a range of different
 * computing languages and to use a range of
 * compilers, including the following:
 * Java: BlueJ
 * C: Miracle C
 * C++: Microsoft Visual C++.NET
 * C#: Microsoft Visual C#.NET
 * Visual Basic: Microsoft Visual Basic.NET
 * PHP: Linux Apache/2.4.18 (Ubuntu) Server 
 */

//variable declarations
//Technically, some of there are not needed in PHP, but it is useful to introduce them here
const RANGE=1000; //we are calculating the prime numbers up to this number. Note that where Java uses final int to declare constants, whereas PHP uses const
$listOfIntegers=array();//Declaring an empty array. This array of integers will hold all the numbers up to the value of RANGE. Mote that this declaration is quite different to the Java array declaration
$root=sqrt(RANGE);//needed later on when removing multiples. Note that PHP prescedes variable name with $, wheras Java does not! Also, PHP square root implementation is simpler that Java.
$product=0; //used later when removing multiples
$printCount=0;//used during print formatting of the program output

//initialising the integer array
for($n=1; $n<=RANGE; $n++)$listOfIntegers[$n]=$n;
/*
//test
for($n=1; $n<=RANGE; $n++){
        echo $listOfIntegers[$n];
}
*/

//starting from n=2, "remove" multiples of n up the value of RANGE
//repeat for increasing values of n up to the square root of RANGE
//This implementation is almost indectical to Java (apart from the $ symbols needed in PHP)
for($n=2; $n<=$root; $n++){
        //delete the multiples of n
        for($multiplier=2; $multiplier*$n<=RANGE; $multiplier++){
            $product=$n*$multiplier;
            $listOfIntegers[$product]=0;
        }
}//end of multiple removal

//print out the prime numbers remaining, remembering that 2 is the first prime number
echo "The Sieve of Eratosthenes program<br/>";
echo "Written in PHP and executed using Linux Apache/2.4.18 (Ubuntu) Server<br/>";
echo "Robin Broad, February 2019.<br/><br/>";
echo "The prime numbers in the range: 1 to ".RANGE." are:-<br/>";//Note that PHP uses the concatenation . here, whereas Java uses string +

//Print the results
for($n=2; $n<=RANGE; $n++){
    //print all non-zero elements of the integer array in blocks of 10 numbers at a time
    if ($listOfIntegers[$n]!=0){
                echo $listOfIntegers[$n]." ";
                $printCount++;
                if($printCount%10==0)echo "<br/>";//every 10 items produces a new line
    }// end if
}//end for

echo "<br/>End of Program.";

/*This program produed the folloing output:
The Sieve of Eratosthenes program
Written in PHP and executed using Linux Apache/2.4.18 (Ubuntu) Server
Robin Broad, February 2019.

The prime numbers in the range: 1 to 1000 are:-
2 3 5 7 11 13 17 19 23 29
31 37 41 43 47 53 59 61 67 71
73 79 83 89 97 101 103 107 109 113
127 131 137 139 149 151 157 163 167 173
179 181 191 193 197 199 211 223 227 229
233 239 241 251 257 263 269 271 277 281
283 293 307 311 313 317 331 337 347 349
353 359 367 373 379 383 389 397 401 409
419 421 431 433 439 443 449 457 461 463
467 479 487 491 499 503 509 521 523 541
547 557 563 569 571 577 587 593 599 601
607 613 617 619 631 641 643 647 653 659
661 673 677 683 691 701 709 719 727 733
739 743 751 757 761 769 773 787 797 809
811 821 823 827 829 839 853 857 859 863
877 881 883 887 907 911 919 929 937 941
947 953 967 971 977 983 991 997
End of Program.
*/
?>