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# atan

(PHP 4, PHP 5)

atanArc tangent

### Description

float atan ( float \$arg )

Returns the arc tangent of arg in radians. atan() is the complementary function of tan(), which means that a==tan(atan(a)) for every value of a that is within atan()'s range.

### Parameters

arg

The argument to process

### Return Values

The arc tangent of arg in radians.

User Contributed Notes
atan
joelperr at kiwi-interactif dot com
12-Jan-2006 01:12
`to obtain the direction of the line, you are better to use the <? atan2((y2-y1)/(x2-x1)) ?> function, since the regular atan function will only return arguments in the half-plane, ie. if y2-y1 and x2-x1 are negative, atan will give you an angle measurement less than 90 degrees, while it really should be  between 180 and 270`
darren_wheatley at hotmail dot com
28-Nov-2003 02:24
```Arc Tan curve manipulation. I used this formula to help with increasing and then diminishing return for y given an increasing x for a game. Ie: Food production (output) is y. Food research is x. The more research you put into x the more you produce, however after a certain point you get less reward.  y = atan(x - pi()) + pi()/2; The + pi()/2 moves it up the y axis so you'd add more if you want it to start higher. The x - pi() moves it to the right so you'd minus more to move it more. If you want stretched along the y axis change it to 2 * atan( ...... ) Dunno how useful it is... but it's there. Daz```
jmartin at columbiaservices dot net
21-Nov-2003 08:10
```I looked for hours trying to come up with a formula to solve the direction that a line was heading (in degrees) when x1,y1 were the starting points, and x2,y2 are the ending points. Here is the equasion I was given, I hope this helps anyone in need of the same one. \$angle = rad2deg(atan2((\$y2 - \$y1), (\$x2 - x1)));```