Trigonometry is an important part of mathematics and PHP’s math functions provides us with some functions which are very useful for calculations involving trigonometry.The tan() function in PHP is used to find the tangent of the argument parameter.
The argument parameter must be in radians.

Syntax:

float tan($value)

Parameters: This function accepts a single parameter $value. It is the number whose tangent value you want to find. The value of this parameter must be in radians.

Return Value: It returns a floating point number which is the tangent value of number passed to it as argument.

Examples:

Input : tan(0.50)  
Output : 0.54630248984379

Input : tan(-0.50) 
Output : -0.54630248984379

Input : tan(5)
Output : -3.3805150062466

Input : tan(M_PI_4) 
Output : 1

Below programs illustrate tan() in PHP:

• Passing 0.50 as a parameter:
  
  

  
  
  

  <?php    echo (tan(0.50));    ?>

  Output:

  0.54630248984379

• Passing -0.50 as a parameter:
  
  

  
  
  

  <?php    echo (tan(-0.50));    ?>

  Output:

  -0.54630248984379

• Passing 5 as a parameter:
  
  

  
  
  

  <?php    echo (tan(5));    ?>

  Output:

  -3.3805150062466

• When (M_PI_4) is passed as a parameter, M_PI_4 is a constant in PHP whose value is 0.78539816339744830962:
  
  

  
  
  

  <?php    echo (tan(M_PI_4));    ?>

  Output:

  1

Reference:
http://php.net/manual/en/function.tan.php