why cant i show correct image from sybas

[netboy]

 I use php4+sybase11+apache,when i select image from sybase($result=sybase_query("select image from imagetable where id=1"),

if size<16k,everything is right,when size>17k,the image is only half(size=16384b).when i use "select datalength(image) from imagetable",the size is 28990b.why cant i read image over 16k?
can some one help me.

 thanks