Call to undefined function pdf_new()

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PHP

Hi all i need to create a report in php using pdf
As i am new to php first i have written a simple program to open a pdf

$pdf = pdf_new();  
pdf_open_file($pdf, "philosophy.pdf");  
pdf_begin_page($pdf, 595, 842);  
$arial = pdf_findfont($pdf, "Arial", "host", 1); pdf_setfont($pdf, $arial, 10); pdf_show_xy($pdf, "There are more things in heaven and earth, Horatio,", 50, 750);
pdf_show_xy($pdf, "than are dreamt of in your philosophy", 50, 730);  
pdf_end_page($pdf);  
pdf_close($pdf); 

But i am getting error as call to undefined function pdf_new(). How to solve this?
Pls help.