Continued fractions with PHP

<?php

// the contfrac(x,y) function outputs an array starting from 0 to y - 1 of all the terms in the continued fraction of x
// due to computational errors, php may output erroneous results after a few terms (eg : using 1/2+sqrt(5)/2 as x doesn't always output ones past 38 terms and sqrt(2) doesn't always output two's after the sole 1 at contfrac(sqrt(2),y)[0] when y >= 20 on my version)

function contfrac($constant, $upto=5) {
    $frac[0] = floor($constant);
    if ($frac[0] - $constant) { $a[0] = 1/($constant-floor($constant)); }
    else { $a[0] = 0; }
    for ($i = 0; $i <= $upto; $i++) {
        if ($a[$i]-floor($a[$i]) != 0) { $a[$i+1] = 1/($a[$i]-floor($a[$i])); }
        else { $a[$i+1] = 0; }
        $frac[$i+1] = floor($a[$i]);
    }
    return $frac;
}

// optional :
$upto = 10;
for ($i = 1; $i > 0; $i++) {
    $u = readline(">");
    $u = str_replace(array("pi()","pi","_&#/!","^"),array("_&#/!","pi()","pi()","**"),$u);
    eval("\$f = " . $u . ";");
    if ($f < 0) { $sgn = "-"; }
    else { $sgn = ""; }
    $u = $sgn . "($u)";
    eval("\$frac = contfrac(" . $u . ", \$upto);");
    print "[$sgn" . $frac[0] . "; " . $frac[1];
    for ($i = 2; $i <= $upto; $i++) {
        print ", " . $frac[$i];
    }
    print ", ...]\n";
}

?>