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<h1>Predefined Constants</h1>
<p class="simpara">
The constants below are always available as part of the PHP core.
</p>
<p class="para">
<table class="doctable table">
<caption><strong>Math constants</strong></caption>
<thead>
<tr>
<th>Constant</th>
<th>Value</th>
<th>Description</th>
<th>Availability</th>
</tr>
</thead>
<tbody class="tbody">
<tr id="constant.m-pi">
<td><strong><code>M_PI</code></strong></td>
<td>3.14159265358979323846</td>
<td>Pi</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-e">
<td><strong><code>M_E</code></strong></td>
<td>2.7182818284590452354</td>
<td>e</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-log2e">
<td><strong><code>M_LOG2E</code></strong></td>
<td>1.4426950408889634074</td>
<td>log_2 e</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-log10e">
<td><strong><code>M_LOG10E</code></strong></td>
<td>0.43429448190325182765</td>
<td>log_10 e</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-ln2">
<td><strong><code>M_LN2</code></strong></td>
<td>0.69314718055994530942</td>
<td>log_e 2</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-ln10">
<td><strong><code>M_LN10</code></strong></td>
<td>2.30258509299404568402</td>
<td>log_e 10</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-pi-2">
<td><strong><code>M_PI_2</code></strong></td>
<td>1.57079632679489661923</td>
<td>pi/2</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-pi-4">
<td><strong><code>M_PI_4</code></strong></td>
<td>0.78539816339744830962</td>
<td>pi/4</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-1-pi">
<td><strong><code>M_1_PI</code></strong></td>
<td>0.31830988618379067154</td>
<td>1/pi</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-2-pi">
<td><strong><code>M_2_PI</code></strong></td>
<td>0.63661977236758134308</td>
<td>2/pi</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-sqrtpi">
<td><strong><code>M_SQRTPI</code></strong></td>
<td>1.77245385090551602729</td>
<td>sqrt(pi)</td>
<td>PHP 5.2.0</td>
</tr>
<tr id="constant.m-2-sqrtpi">
<td><strong><code>M_2_SQRTPI</code></strong></td>
<td>1.12837916709551257390</td>
<td>2/sqrt(pi)</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-sqrt2">
<td><strong><code>M_SQRT2</code></strong></td>
<td>1.41421356237309504880</td>
<td>sqrt(2)</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-sqrt3">
<td><strong><code>M_SQRT3</code></strong></td>
<td>1.73205080756887729352</td>
<td>sqrt(3)</td>
<td>PHP 5.2.0</td>
</tr>
<tr id="constant.m-sqrt1-2">
<td><strong><code>M_SQRT1_2</code></strong></td>
<td>0.70710678118654752440</td>
<td>1/sqrt(2)</td>
<td class="empty"> </td>
</tr>
<tr id="constant.m-lnpi">
<td><strong><code>M_LNPI</code></strong></td>
<td>1.14472988584940017414</td>
<td>log_e(pi)</td>
<td>PHP 5.2.0</td>
</tr>
<tr id="constant.m-euler">
<td><strong><code>M_EULER</code></strong></td>
<td>0.57721566490153286061</td>
<td>Euler constant</td>
<td>PHP 5.2.0</td>
</tr>
<tr id="constant.php-round-half-up">
<td><strong><code>PHP_ROUND_HALF_UP</code></strong></td>
<td>1</td>
<td>Round halves up</td>
<td>PHP 5.3.0</td>
</tr>
<tr id="constant.php-round-half-down">
<td><strong><code>PHP_ROUND_HALF_DOWN</code></strong></td>
<td>2</td>
<td>Round halves down</td>
<td>PHP 5.3.0</td>
</tr>
<tr id="constant.php-round-half-even">
<td><strong><code>PHP_ROUND_HALF_EVEN</code></strong></td>
<td>3</td>
<td>Round halves to even numbers</td>
<td>PHP 5.3.0</td>
</tr>
<tr id="constant.php-round-half-odd">
<td><strong><code>PHP_ROUND_HALF_ODD</code></strong></td>
<td>4</td>
<td>Round halves to odd numbers</td>
<td>PHP 5.3.0</td>
</tr>
<tr id="constant.nan">
<td><strong><code>NAN</code></strong></td>
<td>NAN (as a float)</td>
<td>Not A Number</td>
<td class="empty"> </td>
</tr>
<tr id="constant.inf">
<td><strong><code>INF</code></strong></td>
<td>INF (as a float)</td>
<td>The infinite</td>
<td class="empty"> </td>
</tr>
</tbody>
</table>
</p>
<p class="para">
All constants without a labeled PHP version above are available
starting with PHP 4.0.0.
</p>
</div>
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