I needed the crc32 of a file that was pretty large, so I didn't want to read it into memory.
So I made this:

<?php
    $GLOBALS['__crc32_table']=array();        // Lookup table array
    __crc32_init_table();

    function __crc32_init_table() {            // Builds lookup table array
        // This is the official polynomial used by
        // CRC-32 in PKZip, WinZip and Ethernet.
        $polynomial = 0x04c11db7;

        // 256 values representing ASCII character codes.
        for($i=0;$i <= 0xFF;++$i) {
            $GLOBALS['__crc32_table'][$i]=(__crc32_reflect($i,8) << 24);
            for($j=0;$j < 8;++$j) {
                $GLOBALS['__crc32_table'][$i]=(($GLOBALS['__crc32_table'][$i] << 1) ^
                    (($GLOBALS['__crc32_table'][$i] & (1 << 31))?$polynomial:0));
            }
            $GLOBALS['__crc32_table'][$i] = __crc32_reflect($GLOBALS['__crc32_table'][$i], 32);
        }
    }

    function __crc32_reflect($ref, $ch) {        // Reflects CRC bits in the lookup table
        $value=0;
        
        // Swap bit 0 for bit 7, bit 1 for bit 6, etc.
        for($i=1;$i<($ch+1);++$i) {
            if($ref & 1) $value |= (1 << ($ch-$i));
            $ref = (($ref >> 1) & 0x7fffffff);
        }
        return $value;
    }

    function __crc32_string($text) {        // Creates a CRC from a text string
        // Once the lookup table has been filled in by the two functions above,
        // this function creates all CRCs using only the lookup table.

        // You need unsigned variables because negative values
        // introduce high bits where zero bits are required.
        // PHP doesn't have unsigned integers:
        // I've solved this problem by doing a '&' after a '>>'.

        // Start out with all bits set high.
        $crc=0xffffffff;
        $len=strlen($text);

        // Perform the algorithm on each character in the string,
        // using the lookup table values.
        for($i=0;$i < $len;++$i) {
            $crc=(($crc >> 8) & 0x00ffffff) ^ $GLOBALS['__crc32_table'][($crc & 0xFF) ^ ord($text{$i})];
        }
        
        // Exclusive OR the result with the beginning value.
        return $crc ^ 0xffffffff;
    }
    
    function __crc32_file($name) {            // Creates a CRC from a file
        // Info: look at __crc32_string

        // Start out with all bits set high.
        $crc=0xffffffff;

        if(($fp=fopen($name,'rb'))===false) return false;

        // Perform the algorithm on each character in file
        for(;;) {
            $i=@fread($fp,1);
            if(strlen($i)==0) break;
            $crc=(($crc >> 8) & 0x00ffffff) ^ $GLOBALS['__crc32_table'][($crc & 0xFF) ^ ord($i)];
        }
        
        @fclose($fp);
        
        // Exclusive OR the result with the beginning value.
        return $crc ^ 0xffffffff;
    }
?>