图片格式查看：
用十六进制编辑器察看过图片的文件头，分析的结果，供大家参考：
1.JPEG/JPG
- 文件头标识 (2 bytes): $ff, $d8 (SOI) (JPEG 文件标识)
- 文件结束标识 (2 bytes): $ff, $d9 (EOI)
2.TGA
- 未压缩的前5字节 00 00 02 00 00
- RLE压缩的前5字节 00 00 10 00 00
3.PNG
- 文件头标识 (8 bytes) 89 50 4E 47 0D 0A 1A 0A
4.GIF
- 文件头标识 (6 bytes) 47 49 46 38 39(37) 61
G I F 8 9 (7) a
gif89a 有透明
gif87a 无透明
5.BMP
- 文件头标识 (2 bytes) 42 4D
B M
6.PCX
- 文件头标识 (1 bytes) 0A
7.TIFF
- 文件头标识 (2 bytes) 4D 4D 或 49 49
8.ICO
- 文件头标识 (8 bytes) 00 00 01 00 01 00 20 20
9.CUR
- 文件头标识 (8 bytes) 00 00 02 00 01 00 20 20
10.IFF
- 文件头标识 (4 bytes) 46 4F 52 4D
F O R M
11.ANI
- 文件头标识 (4 bytes) 52 49 46 46
R I F F
根据这些文件头标识的收集，我可以写一个识别图像格式的模块了。但是在写这个模块之前可以对收集到的文件头标识进行优化，使得程序中字符串比对次数尽量的少。
1.JPEG我们知需要比对文件头的$ff, $d8这两个字符，而不用读取最后的两个结束标识了。
2.TGA，ICO，CUR只需比对第三个与第五个字符即可。
3.PNG比对[89][50]这两个字符。
4.GIF比对[47][49][46]与第五个字符。
废话不多说了，利用内存流来判断文件的格式，其实判断文件的前几个字节就可以简单的判断这个文件是什么类型的文件，例如
jpg文件 是 FFD8 (从低位到高位就要反过来 D8FF 下面都是一样)
BMP文件 是 424D ---4D42
附：常见文件的文件头标识
255044 PDF
526563 EML
D0CF11 PPT
4D5AEE COM
E93B03 COM
4D5A90 EXE
424D3E BMP
49492A TIF
384250 PSD
C5D0D3 EPS
0A0501 PCS
89504E PNG
060500 RAW
000002 TGA
60EA27 ARJ
526172 RAR
504B03 ZIP
495363 CAB
1F9D8C Z
524946 WAV
435753 SWF
3026B2 WMV
3026B2 WMA
2E524D RM
00000F MOV
000077 MOV
000001 MPA
FFFB50 MP3
234558 m3u
3C2144 HTM
FFFE3C XSL
3C3F78 XML
3C3F78 MSC
4C0000 LNK
495453 CHM
805343 scm
D0CF11 XLS
31BE00 WRI
00FFFF MDF
4D4544 MDS
5B436C CCD
00FFFF IMG
FFFFFF SUB
17A150 PCB
2A5052 ECO
526563 PPC
000100 DDB
42494C LDB
2A7665 SCH
2A2420 LIB
434841 FNT
7B5C72 RTF
7B5072 GTD
234445 PRG
000007 PJT
202020 BAS
000002 TAG
4D5A90 dll
4D5A90 OCX
4D5A50 DPL
3F5F03 HLP
4D5A90 OLB
4D5A90 IMM
4D5A90 IME
3F5F03 LHP
C22020 NLS
5B5769 CPX
4D5A16 DRV
5B4144 PBK
24536F PLL
4E4553 NES
87F53E GBC
00FFFF SMD
584245 XBE
005001 XMV
000100 TTF
484802 PDG
000100 TST
414331 dwg
D0CF11 max
另外还有一些重要的文件，没有固定的文件头，如下：
TXT 没固定文件头定义
TMP 没固定文件头定义
INI 没固定文件头定义
BIN 没固定文件头定义
DBF 没固定文件头定义
C 没没固定文件头定义
CPP 没固定文件头定义
H 没固定文件头定义
BAT 没固定文件头定义
还有一些不同的文件有相同的文件头，最典型的就是下面：
4D5A90 EXE
4D5A90 dll
4D5A90 OCX
4D5A90 OLB
4D5A90 IMM
4D5A90 IME
附网上例子
根据二进制流获取图片类型
public function getFileType($file,&$fileType,&$mime){
$bin = substr($file,0,2);
$strInfo = @unpack("C2chars", $bin);
$typeCode = intval($strInfo['chars1'].$strInfo['chars2']);
$fileType = '';
switch ($typeCode)
{
case 7790:
$fileType = 'exe';
return false;
break;
case 7784:
$fileType = 'midi';
return false;
break;
case 8297:
$fileType = 'rar';
return false;
break;
case 255216:
$fileType = 'jpg';
$mime = 'image/jpeg';
return true;
break;
case 7173:
$fileType = 'gif';
$mime = 'image/gif';
return true;
break;
case 6677:
$fileType = 'bmp';
$mime = 'image/bmp';
return true;
break;
case 13780:
$fileType = 'png';
$mime = 'image/png';
return true;
break;
default:
return false;
break;
}
return false;
}
mmime type:
ime.ez=application/andrew-inset
mime.csm=application/cu-seeme
mime.cu=application/cu-seeme
mime.tsp=application/dsptype
mime.spl=application/futuresplash
mime.hta=application/hta
mime.cpt=application/mac-compactpro
mime.hqx=application/mac-binhex40
mime.nb=application/mathematica
mime.mdb=application/msaccess
mime.doc=application/msword
mime.dot=application/msword
mime.bin=application/octet-stream
mime.oda=application/oda
mime.ogg=application/ogg
mime.prf=application/pics-rules
mime.key=application/pgp-keys
mime.pdf=application/pdf
mime.pgp=application/pgp-signature
mime.ps=application/postscript
mime.ai=application/postscript
mime.eps=application/postscript
mime.rss=application/rss+xml
mime.rtf=application/rtf
mime.smi=application/smil
mime.smil=application/smil
mime.wp5=application/wordperfect5.1
mime.xht=application/xhtml+xml
mime.xhtml=application/xhtml+xml
mime.zip=application/zip
mime.cdy=application/vnd.cinderella
mime.mif=application/vnd.mif
mime.xls=application/vnd.ms-excel
mime.xlb=application/vnd.ms-excel
mime.cat=application/vnd.ms-pki.seccat
mime.stl=application/vnd.ms-pki.stl
mime.ppt=application/vnd.ms-powerpoint
mime.pps=application/vnd.ms-powerpoint
mime.pot=application/vnd.ms-powerpoint
mime.sdc=application/vnd.stardivision.calc
mime.sda=application/vnd.stardivision.draw
mime.sdd=application/vnd.stardivision.impress
mime.sdp=application/vnd.stardivision.impress
mime.smf=application/vnd.stardivision.math
mime.sdw=application/vnd.stardivision.writer
mime.vor=application/vnd.stardivision.writer
mime.sgl=application/vnd.stardivision.writer-global
mime.sxc=application/vnd.sun.xml.calc
mime.stc=application/vnd.sun.xml.calc.template
mime.sxd=application/vnd.sun.xml.draw
mime.std=application/vnd.sun.xml.draw.template
mime.sxi=application/vnd.sun.xml.impress
mime.sti=application/vnd.sun.xml.impress.template
mime.sxm=application/vnd.sun.xml.math
mime.sxw=application/vnd.sun.xml.writer
mime.sxg=application/vnd.sun.xml.writer.global
mime.stw=application/vnd.sun.xml.writer.template
mime.sis=application/vnd.symbian.install
mime.wbxml=application/vnd.wap.wbxml
mime.wmlc=application/vnd.wap.wmlc
mime.wmlsc=application/vnd.wap.wmlscriptc
mime.wk=application/x-123
mime.dmg=application/x-apple-diskimage
mime.bcpio=application/x-bcpio
mime.torrent=application/x-bittorrent
mime.cdf=application/x-cdf
mime.vcd=application/x-cdlink
mime.pgn=application/x-chess-pgn
mime.cpio=application/x-cpio
mime.csh=application/x-csh
mime.deb=application/x-debian-package
mime.dcr=application/x-director
mime.dir=application/x-director
mime.dxr=application/x-director
mime.wad=application/x-doom
mime.dms=application/x-dms
mime.dvi=application/x-dvi
mime.pfa=application/x-font
mime.pfb=application/x-font
mime.gsf=application/x-font
mime.pcf=application/x-font
mime.pcf.Z=application/x-font
mime.spl=application/x-futuresplash
mime.gnumeric=application/x-gnumeric
mime.sgf=application/x-go-sgf
mime.gcf=application/x-graphing-calculator
mime.gtar=application/x-gtar
mime.tgz=application/x-gtar
mime.taz=application/x-gtar
mime.hdf=application/x-hdf
mime.phtml=application/x-httpd-php
mime.pht=application/x-httpd-php
mime.php=application/x-httpd-php
mime.phps=application/x-httpd-php-source
mime.php3=application/x-httpd-php3
mime.php3p=application/x-httpd-php3-preprocessed
mime.php4=application/x-httpd-php4
mime.ica=application/x-ica
mime.ins=application/x-internet-signup
mime.isp=application/x-internet-signup
mime.iii=application/x-iphone
mime.jar=application/x-java-archive
mime.jnlp=application/x-java-jnlp-file
mime.ser=application/x-java-serialized-object
mime.class=application/x-java-vm
mime.js=application/x-javascript
mime.chrt=application/x-kchart
mime.kil=application/x-killustrator
mime.kpr=application/x-kpresenter
mime.kpt=application/x-kpresenter
mime.skp=application/x-koan
mime.skd=application/x-koan
mime.skt=application/x-koan
mime.skm=application/x-koan
mime.ksp=application/x-kspread
mime.kwd=application/x-kword
mime.kwt=application/x-kword
mime.latex=application/x-latex
mime.lha=application/x-lha
mime.lzh=application/x-lzh
mime.lzx=application/x-lzx
mime.frm=application/x-maker
mime.maker=application/x-maker
mime.frame=application/x-maker
mime.fm=application/x-maker
mime.fb=application/x-maker
mime.book=application/x-maker
mime.fbdoc=application/x-maker
mime.mif=application/x-mif
mime.wmz=application/x-ms-wmz
mime.wmd=application/x-ms-wmd
mime.com=application/x-msdos-program
mime.exe=application/x-msdos-program
mime.bat=application/x-msdos-program
mime.dll=application/x-msdos-program
mime.msi=application/x-msi
mime.nc=application/x-netcdf
mime.pac=application/x-ns-proxy-autoconfig
mime.nwc=application/x-nwc
mime.o=application/x-object
mime.oza=application/x-oz-application
mime.pl=application/x-perl
mime.pm=application/x-perl
mime.p7r=application/x-pkcs7-certreqresp
mime.crl=application/x-pkcs7-crl
mime.qtl=application/x-quicktimeplayer
mime.rpm=application/x-redhat-package-manager
mime.shar=application/x-shar
mime.swf=application/x-shockwave-flash
mime.swfl=application/x-shockwave-flash
mime.sh=application/x-sh
mime.sit=application/x-stuffit
mime.sv4cpio=application/x-sv4cpio
mime.sv4crc=application/x-sv4crc
mime.tar=application/x-tar
mime.tcl=application/x-tcl
mime.tex=application/x-tex
mime.gf=application/x-tex-gf
mime.pk=application/x-tex-pk
mime.texinfo=application/x-texinfo
mime.texi=application/x-texinfo
mime.~=application/x-trash
mime.%=application/x-trash
mime.bak=application/x-trash
mime.old=application/x-trash
mime.sik=application/x-trash
mime.t=application/x-troff
mime.tr=application/x-troff
mime.roff=application/x-troff
mime.man=application/x-troff-man
mime.me=application/x-troff-me
mime.ms=application/x-troff-ms
mime.ustar=application/x-ustar
mime.src=application/x-wais-source
mime.wz=application/x-wingz
mime.crt=application/x-x509-ca-cert
mime.fig=application/x-xfig
mime.au=audio/basic
mime.snd=audio/basic
mime.mid=audio/midi
mime.midi=audio/midi
mime.kar=audio/midi
mime.mpga=audio/mpeg
mime.mpega=audio/mpeg
mime.mp2=audio/mpeg
mime.mp3=audio/mpeg
mime.m3u=audio/mpegurl
mime.sid=audio/prs.sid
mime.aif=audio/x-aiff
mime.aiff=audio/x-aiff
mime.aifc=audio/x-aiff
mime.gsm=audio/x-gsm
mime.m3u=audio/x-mpegurl
mime.wma=audio/x-ms-wma
mime.wax=audio/x-ms-wax
mime.rpm=audio/x-pn-realaudio-plugin
mime.ra=audio/x-pn-realaudio
mime.rm=audio/x-pn-realaudio
mime.ram=audio/x-pn-realaudio
mime.ra=audio/x-realaudio
mime.pls=audio/x-scpls
mime.sd2=audio/x-sd2
mime.wav=audio/x-wav
mime.pdb=chemical/x-pdb
mime.xyz=chemical/x-xyz
mime.bmp=image/bmp
mime.gif=image/gif
mime.ief=image/ief
mime.jpeg=image/jpeg
mime.jpg=image/jpeg
mime.jpe=image/jpeg
mime.pcx=image/pcx
mime.png=image/png
mime.svg=image/svg+xml
mime.svgz=image/svg+xml
mime.tiff=image/tiff
mime.tif=image/tiff
mime.wbmp=image/vnd.wap.wbmp
mime.ras=image/x-cmu-raster
mime.cdr=image/x-coreldraw
mime.pat=image/x-coreldrawpattern
mime.cdt=image/x-coreldrawtemplate
mime.cpt=image/x-corelphotopaint
mime.djvu=image/x-djvu
mime.djv=image/x-djvu
mime.ico=image/x-icon
mime.art=image/x-jg
mime.jng=image/x-jng
mime.bmp=image/x-ms-bmp
mime.psd=image/x-photoshop
mime.pnm=image/x-portable-anymap
mime.pbm=image/x-portable-bitmap
mime.pgm=image/x-portable-graymap
mime.ppm=image/x-portable-pixmap
mime.rgb=image/x-rgb
mime.xbm=image/x-xbitmap
mime.xpm=image/x-xpixmap
mime.xwd=image/x-xwindowdump
mime.igs=model/iges
mime.iges=model/iges
mime.msh=model/mesh
mime.mesh=model/mesh
mime.silo=model/mesh
mime.wrl=model/vrml
mime.vrml=model/vrml
mime.csv=text/comma-separated-values
mime.css=text/css
mime.323=text/h323
mime.htm=text/html
mime.html=text/html
mime.uls=text/iuls
mime.mml=text/mathml
mime.asc=text/plain
mime.txt=text/plain
mime.text=text/plain
mime.diff=text/plain
mime.rtx=text/richtext
mime.rtf=text/rtf
mime.sct=text/scriptlet
mime.wsc=text/scriptlet
mime.tm=text/texmacs
mime.ts=text/texmacs
mime.tsv=text/tab-separated-values
mime.jad=text/vnd.sun.j2me.app-descriptor
mime.wml=text/vnd.wap.wml
mime.wmls=text/vnd.wap.wmlscript
mime.xml=text/xml
mime.xsl=text/xml
mime.h++=text/x-c++hdr
mime.hpp=text/x-c++hdr
mime.hxx=text/x-c++hdr
mime.hh=text/x-c++hdr
mime.c++=text/x-c++src
mime.cpp=text/x-c++src
mime.cxx=text/x-c++src
mime.cc=text/x-c++src
mime.h=text/x-chdr
mime.csh=text/x-csh
mime.c=text/x-csrc
mime.java=text/x-java
mime.moc=text/x-moc
mime.p=text/x-pascal
mime.pas=text/x-pascal
mime.***=text/x-pcs-***
mime.shtml=text/x-server-parsed-html
mime.etx=text/x-setext
mime.sh=text/x-sh
mime.tcl=text/x-tcl
mime.tk=text/x-tcl
mime.tex=text/x-tex
mime.ltx=text/x-tex
mime.sty=text/x-tex
mime.cls=text/x-tex
mime.vcs=text/x-vcalendar
mime.vcf=text/x-vcard
mime.dl=video/dl
mime.fli=video/fli
mime.gl=video/gl
mime.mpeg=video/mpeg
mime.mpg=video/mpeg
mime.mpe=video/mpeg
mime.qt=video/quicktime
mime.mov=video/quicktime
mime.mxu=video/vnd.mpegurl
mime.dif=video/x-dv
mime.dv=video/x-dv
mime.lsf=video/x-la-asf
mime.lsx=video/x-la-asf
mime.mng=video/x-mng
mime.asf=video/x-ms-asf
mime.asx=video/x-ms-asf
mime.wm=video/x-ms-wm
mime.wmv=video/x-ms-wmv
mime.wmx=video/x-ms-wmx
mime.wvx=video/x-ms-wvx
mime.avi=video/x-msvideo
mime.movie=video/x-sgi-movie
mime.ice=x-conference/x-cooltalk
mime.vrm=x-world/x-vrml
mime.vrml=x-world/x-vrml
mime.wrl=x-world/x-vrml
参考：
http://www.cnblogs.com/jetlee/archive/2010/11/05/1870223.html
http://blog.sina.com.cn/s/blog_6bfe46bc0100q1bx.html
http://samver.blogbus.com/logs/60772235.html
http://www.garykessler.net/library/file_sigs.html