exec ("ffmpeg -f concat -i files.txt -c  copy finalvideo.mp4");

exec ("ffmpeg -i finalvideo.mp4 -c:v copy -c:a libfaac -r 24 output.mp4");

/home/fresh/public_html/ffmpeg/output$ ffmpeg -i /home/fresh/public_html/ffmpeg/output/ENP22122.mp4
ffmpeg version N-61053-gab5d46d-syslint Copyright (c) 2000-2014 the FFmpeg developers
built on Mar  3 2014 13:27:18 with gcc 4.4.7 (GCC) 20120313 (Red Hat 4.4.7-4)
configuration: --prefix=/usr/local/cpffmpeg --enable-shared --enable-nonfree --enable-gpl --enable-pthreads --enable-libopencore-amrnb --enable-decoder=liba52 --enable-  libopencore-amrwb --enable-libfaac --enable-libmp3lame --enable-libtheora --enable-  libvorbis --enable-libx264 --enable-libxvid --extra-cflags=-I/usr/local/cpffmpeg/include/ --extra-ldflags=-L/usr/local/cpffmpeg/lib --enable-version3 --extra-version=syslint
libavutil      52. 66.100 / 52. 66.100
libavcodec     55. 52.102 / 55. 52.102
libavformat    55. 33.100 / 55. 33.100
libavdevice    55. 10.100 / 55. 10.100
libavfilter     4.  2.100 /  4.  2.100
libswscale      2.  5.101 /  2.  5.101
libswresample   0. 18.100 /  0. 18.100
libpostproc    52.  3.100 / 52.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/home/fresh/public_html/ffmpeg/output/ENP22122.mp4':
Metadata:
major_brand     : isom
minor_version   : 512
compatible_brands: isomiso2avc1mp41
encoder         : Lavf55.33.100
Duration: 00:00:04.07, start: 0.000000, bitrate: 63 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 480x360 [SAR 1:1   DAR 4:3], 5 kb/s, 15 fps, 15 tbr, 15360 tbn, 30 tbc (default)
Metadata:
  handler_name    : VideoHandler
Stream #0:1(und): Audio: mp3 (mp4a / 0x6134706D), 44100 Hz, mono, s16p, 96 kb/s (default)
Metadata:
  handler_name    : SoundHandler

/home/fresh/public_html/ffmpeg/output$ ffmpeg -i /home/fresh/public_html/ffmpeg/output/ENT22122.mp4
ffmpeg version N-61053-gab5d46d-syslint Copyright (c) 2000-2014 the FFmpeg developers
built on Mar  3 2014 13:27:18 with gcc 4.4.7 (GCC) 20120313 (Red Hat 4.4.7-4)
configuration: --prefix=/usr/local/cpffmpeg --enable-shared --enable-nonfree --enable-gpl --enable-pthreads --enable-libopencore-amrnb --enable-decoder=liba52 --enable-  libopencore-amrwb --enable-libfaac --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --extra-cflags=-I/usr/local/cpffmpeg/include/ --extra-ldflags=-L/usr/local/cpffmpeg/lib --enable-version3 --extra-version=syslint
libavutil      52. 66.100 / 52. 66.100
libavcodec     55. 52.102 / 55. 52.102
libavformat    55. 33.100 / 55. 33.100
libavdevice    55. 10.100 / 55. 10.100
libavfilter     4.  2.100 /  4.  2.100
libswscale      2.  5.101 /  2.  5.101
libswresample   0. 18.100 /  0. 18.100
libpostproc    52.  3.100 / 52.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/home/fresh/public_html/ffmpeg/output/ENT22122.mp4':
Metadata:
major_brand     : isom
minor_version   : 512
compatible_brands: isomiso2avc1mp41
encoder         : Lavf55.33.100
Duration: 00:00:03.07, start: 0.000000, bitrate: 56 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 480x360 [SAR 1:1 DAR 4:3], 6 kb/s, 15 fps, 15 tbr, 15360 tbn, 30 tbc (default)
Metadata:
  handler_name    : VideoHandler
Stream #0:1(und): Audio: mp3 (mp4a / 0x6134706D), 44100 Hz, mono, s16p, 96 kb/s (default)
Metadata:
  handler_name    : SoundHandler

/home/fresh/public_html/ffmpeg/output$ ffmpeg -f concat -i /home/fresh/public_html   /ffmpeg/filesnotime.txt -c  copy /home/fresh/public_html/ffmpeg/finalvideo.mp4
ffmpeg version N-61053-gab5d46d-syslint Copyright (c) 2000-2014 the FFmpeg developers
built on Mar  3 2014 13:27:18 with gcc 4.4.7 (GCC) 20120313 (Red Hat 4.4.7-4)
configuration: --prefix=/usr/local/cpffmpeg --enable-shared --enable-nonfree --enable-gpl --enable-pthreads --enable-libopencore-amrnb --enable-decoder=liba52 --enable-libopencore-amrwb --enable-libfaac --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libx264 --enable-libxvid --extra-cflags=-I/usr/local/cpffmpeg/include/ --extra-ldflags=-L/usr/local/cpffmpeg/lib --enable-version3 --extra-version=syslint
libavutil      52. 66.100 / 52. 66.100
libavcodec     55. 52.102 / 55. 52.102
libavformat    55. 33.100 / 55. 33.100
libavdevice    55. 10.100 / 55. 10.100
libavfilter     4.  2.100 /  4.  2.100
libswscale      2.  5.101 /  2.  5.101
libswresample   0. 18.100 /  0. 18.100
libpostproc    52.  3.100 / 52.  3.100
[concat @ 0x14a2900] Estimating duration from bitrate, this may be inaccurate
Input #0, concat, from '/home/fresh/public_html/ffmpeg/filesnotime.txt':
Duration: 00:00:00.08, start: 0.000000, bitrate: 102 kb/s
Stream #0:0: Video: h264 (High) (avc1 / 0x31637661), yuv420p, 480x360 [SAR 1:1 DAR 4:3], 6 kb/s, 15 fps, 15 tbr, 15360 tbn, 30 tbc
Stream #0:1: Audio: mp3 (mp4a / 0x6134706D), 44100 Hz, mono, s16p, 96 kb/s
Output #0, mp4, to '/home/fresh/public_html/ffmpeg/finalvideo.mp4':
Metadata:
encoder         : Lavf55.33.100
Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 480x360 [SAR 1:1 DAR 4:3], q=2-31, 6 kb/s, 15 fps, 15360 tbn, 15360 tbc
Stream #0:1: Audio: mp3 (i[0][0][0] / 0x0069), 44100 Hz, mono, 96 kb/s
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #0:1 -> #0:1 (copy)

frame=  828 fps=0.0 q=-1.0 Lsize=     392kB time=00:00:55.00 bitrate=  58.4kbits/s
video:81kB audio:289kB subtitle:0 data:0 global headers:0kB muxing overhead 6.031527%

ffmpeg -f lavfi -i aevalsrc=0 -i ENP22122.mp4 -shortest
 -c:v copy -c:a aac -strict -2 newmuxedfile.mp4

ffmpeg -i ENA22122.mp4 -i newmuxedfile.mp4 -filter_complex
'[0:0] [0:1] [1:0] [1:1] concat=n=2:v=1:a=1 [v] [a]'
-map '[v]' -map '[a]' -c:v libx264 -c:a aac -strict -2 output.mp4