Using pdf_translate()
The pdf_translate() function is used to move the origin of the PDF page (usually the lower-left hand corner) somewhere else.

Another way to draw a circle
<?php
    /* Save the current state */
    pdf_save($pdf_r);

    /* Move the orgin from the lower-left to the center of the document */
    pdf_translate($pdf_r, PAGE_WIDTH/2, PAGE_HEIGHT/2);
    
    /* Draw a circle at 0,0 (now the center of the document)*/
    pdf_circle($pdf_r, 0, 0, 100);
    
    pdf_stroke($pdf_r);
    
    /* Restore the coordinate system */
    pdf_restore($pdf_r);
?>