I previously blogged about this Project Euler puzzle 6 years ago, using ColdFusion. This is my approach using PHP as a simple practical exercise for myself, and I’d appreciate any feedback on my PHP code.

Problem 8:

The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

…

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

This question has been slightly changed since I originally did this. Previously it was the product of 5 adjacent digits; now it’s the product of 13 adjacent digits.

<?php
$digits = <<<FOO
73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450
FOO;
$max = 0;
$limit = 13;
// turn this into a single-line string
$digits = str_replace(["\n", "\r"], "", $digits);
for ($i = 0; $i < strlen($digits); $i++) {
$sum = 1;
// multiply together our 13 digits
for ($j = 0; $j < $limit; $j++) {
$sum *= substr($digits, $i + $j, 1);
}
if ($sum > $max) {
$max = $sum;
}
}
echo $max;

So I’m using the Heredoc syntax, equivalent to cfsavecontent, for a multi-line string. I’m not sure if it would be advantageous in this case to use Nowdoc, or even just appending each line to a single string, but for now Heredoc seems simple and does what I’m looking for.

I expect there’s some de facto convention for what to use for the end identifier (the bit after the <<< and again at the end of the string), but for now FOO will suffice. Is it covered in any of the PSR coding standards?

So I get rid of the newline and carriage return characters from the string, turning it into one single line string. Initially I tried doing *str_replace(['\n', '\r'], “”, $digits)* except that didn’t work because it didn’t like the single quoted characters. Instead I had to double-quote them, obviously!

Again we find inconsistencies in PHP, this time with the naming of the functions, *str_replace()* but *strlen()*, being the equivalent of ColdFusion’s *replace()* and *len()*.

And *substr(string, start[, count])* being more fully-featured than CFML’s *mid(string, start, count). *If you use negative numbers it can extract starting from the end of the string. And if you don’t specify the *count* parameter, it goes up to the end of the string.