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    <h1>Introduction</h1> <p>Floating-point arithmetic is considered an esoteric subject by many people. This is rather surprising because floating-point is ubiquitous in computer systems. Most fractional numbers don't have an exact representation as a binary fraction, so there is some rounding going on. A good start is <a href="http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html" rel="noreferrer">What Every Computer Scientist Should Know About Floating-Point Arithmetic</a></p> <h2>Questions</h2> <h3>Question 1</h3> <blockquote> <p>Can I rely on this solution if I need just <strong>precise 2 digits calculations</strong> (money)?</p> </blockquote> <h3>Answer 1</h3> <p>If you need need <strong>precise 2 digits</strong> then the answer is <strong>NO</strong> you can not use the php precision settings to ascertain a <strong>2 digit</strong> decimal all the time even if you are <code>not going to work on numbers higher than 10^6</code>. </p> <p>During calculations there is possibility that the precision length can be increased if the length is less than 8</p> <h3>Question 2</h3> <blockquote> <p>If not can you provide me a clear example when this solutions fails?</p> </blockquote> <h3>Answer 2</h3> <pre><code>ini_set('precision', 8); // your precision $a = 5.88 ; // cost of 1kg $q = 2.49 ;// User buys 2.49 kg $b = $a * 0.01 ; // 10% Discount only on first kg ; echo ($a * $q) - $b; </code></pre> <p>Output </p> <pre><code>14.5824 &lt;---- not precise 2 digits calculations even if precision is 8 </code></pre> <h3>Question 3</h3> <blockquote> <p>Which php.ini.precision value suits best two digits, money calculations?</p> </blockquote> <h3>Answer 3</h3> <p>Precision and Money calculation are 2 different things ... it's not a good idea to use PHP precision for as a base for your financial calculations or floating point length </p> <h2>Simple Test</h2> <p>Lest Run some example together using <code>bcmath</code> , <code>number_format</code> and simple <code>minus</code></p> <p><strong><code>Base</code></strong> </p> <pre><code>$a = 342349.23; $b = 341765.07; </code></pre> <p><strong><code>Example A</code></strong></p> <pre><code>ini_set('precision', 20); // set to 20 echo $a - $b, PHP_EOL; echo floatval(round($a - $b, 2)), PHP_EOL; echo number_format($a - $b, 2), PHP_EOL; echo bcsub($a, $b, 2), PHP_EOL; </code></pre> <p>Output </p> <pre><code>584.15999999997438863 584.15999999999996817 &lt;----- Round having a party 584.16 584.15 &lt;-------- here is 15 because precision value is 20 </code></pre> <p><strong><code>Example B</code></strong></p> <pre><code>ini_set('precision', 14); // change to 14 echo $a - $b, PHP_EOL; echo floatval(round($a - $b, 2)), PHP_EOL; echo number_format($a - $b, 2), PHP_EOL; echo bcsub($a, $b, 2), PHP_EOL; </code></pre> <p>Output </p> <pre><code>584.15999999997 584.16 584.16 584.16 &lt;-------- at 14 it changed to 16 </code></pre> <p><strong><code>Example C</code></strong></p> <pre><code>ini_set('precision', 6); // change to 6 echo $a - $b, PHP_EOL; echo floatval(round($a - $b, 2)), PHP_EOL; echo number_format($a - $b, 2), PHP_EOL; echo bcsub($a, $b, 2), PHP_EOL; </code></pre> <p>Output </p> <pre><code>584.16 584.16 584.16 584.00 &lt;--- at 6 it changed to 00 </code></pre> <p><strong><code>Example D</code></strong></p> <pre><code>ini_set('precision', 3); // change to 3 echo $a - $b, PHP_EOL; echo floatval(round($a - $b, 2)), PHP_EOL; echo number_format($a - $b, 2), PHP_EOL; echo bcsub($a, $b, 2), PHP_EOL; </code></pre> <p>Output </p> <pre><code>584 584 584.16 &lt;-------------------------------- They only consistent value 0.00  &lt;--- at 3 .. everything is gone </code></pre> <h2>Conclusion</h2> <p>Forget about floating point and just calculate in <code>cents</code> then later divided by <code>100</code> if that is too late just simply use <code>number_format</code> it looks consistent to me .</p> <h1>Update</h1> <blockquote> <p>Question 1: Is precision workaround gonna fail for numbers between 0..999999.99, where A and B is a number with decimal places? If so please provide me an example</p> </blockquote> <p>Form <code>0</code> to <code>999999.99</code> at increment of of <code>0.01</code> is about <code>99,999,999</code> the combination possibility of your loop is <code>9,999,999,800,000,000</code> I really don't think anyone would want to run such test for you.</p> <p>Since floating point are binary numbers with finite precision trying to set <code>precision</code> would have limited effect to ensure accuracy Here is a simple test :</p> <pre><code>ini_set('precision', 8); $a = 0.19; $b = 0.16; $c = 0.01; $d = 0.01; $e = 0.01; $f = 0.01; $g = 0.01; $h = $a + $b + $c + $d + $e + $f + $g; echo "Total: " , $h , PHP_EOL; $i = $h-$a; $i = $i-$b; $i = $i-$c; $i = $i-$d; $i = $i-$e; $i = $i-$f; $i = $i-$g; echo $i , PHP_EOL; </code></pre> <p>Output </p> <pre><code>Total: 0.4 1.0408341E-17 &lt;--- am sure you would expect 0.00 here ; </code></pre> <p>Try </p> <pre><code>echo round($i,2) , PHP_EOL; echo number_format($i,2) , PHP_EOL; </code></pre> <p>Output </p> <pre><code>0 0.00 &lt;------ still confirms number_format is most accurate to maintain 2 digit </code></pre> <blockquote> <p>Question 2: How to estimate/calculate when precision workaround fails? Without such crazy tests? Is there any mathematical*, straight answer for it? How to calculate is gonna to fail or not?</p> </blockquote> <p>The fact sill remains <a href="http://en.wikipedia.org/wiki/Floating_point" rel="noreferrer">Floating Point</a> have <a href="http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems" rel="noreferrer">Accuracy Problems</a> but for mathematical solutions you can look at</p> <ul> <li><a href="http://en.wikipedia.org/wiki/Floating_point#Machine_precision_and_backward_error_analysis" rel="noreferrer">Machine precision and backward error analysis</a></li> <li><a href="http://en.wikipedia.org/wiki/Floating_point#Minimizing_the_effect_of_accuracy_problems" rel="noreferrer">Minimizing the effect of accuracy problems</a></li> </ul> <blockquote> <p>i don't need to know floating point calculations works, but when workaround fails if you know precision, and range of A and B</p> </blockquote> <p><img src="https://upload.wikimedia.org/math/a/3/0/a302c220e330c52c50dbedeb7ff8859b.png" alt="enter image description here"></p> <p>Not sure what that statement means :)</p>
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    1. POCan I rely on PHP php.ini precision workaround for floating point issue
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    1. CONice one, thank you. But regarding **Answer 2** - still there is no example when solution fails on 2 digits number add/substract calculations like `342349.23 - 341765.07 = 584.15999999992`. And in **Example C** precision workaround is acctionaly.. doing the proper job :)
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    2. COAnd still it looks like `precision=8` is good chocie for me
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    3. COMy Belive is that you need exactly 2 digit `584.15999999992` does not look like it
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