ffmpeg images to video with different start times and durations - command-line

Question

I've recently learned of FFMPEG's existence and I am trying to use it on my wordpress site.
On the site I am working on a html/php/js form page that lets users upload pictures, and set when the image shows and for how long.
Right now the code I have is only showing one image for the entire video.
<?php if (isset($_POST['button'])) {
echo shell_exec('ffmpeg -t '.$cap_1.' -i /myurl/beach-1866431.jpg -t '.$cap_2.' -i /myurl/orlando-1104481-1.jpg -filter_complex "scale=1280:-2" -i /myurl/audio.mp3 -c:v libx264 -pix_fmt yuv420p -t 30 -y /myurl/'.$v_title.'.mp4 2>&1');
} ?>
I tried setting "-t" for the duration with my php variables but nothing changes and I cant figure out what to use for the start time of each image.
Also, when writing shell_exec commands, instead of it all being on one line, is there a way to write working command code in php files with line breaks? For example -
<?php if (isset($_POST['button'])) {
echo shell_exec('ffmpeg -t '.$cap_1.' -i /myurl/beach-1866431.jpg
-t '.$cap_2.' -i /myurl/orlando-1104481-1.jpg
-filter_complex "scale=1280:-2"
-i /myurl/audio.mp3
-c:v libx264 -pix_fmt yuv420p -t 30 -y /myurl/'.$v_title.'.mp4 2>&1');
} ?>
EDIT
So far the concat text file seems to work, however I do not know how to set the start times for each image ---
ffconcat version 1.0
file /path/beach-1866431.jpg
duration 3
file /path/orlando-1104481-1.jpg
duration 5
file /path/beach-1866431.jpg
And ffmpeg command -
shell_exec('ffmpeg -f concat -safe 0 -i /path/file.txt -filter_complex "scale=1280:-2" -i /path/audio.mp3 -c:v libx264 -pix_fmt yuv420p -t 30 -y /path/'.$v_title.'.mp4 2>&1');
EDIT 2
Using the concat method suggested, my code now looks like this --
<?php if (isset($_POST['button'])) {
echo shell_exec('ffmpeg \
-loop 1 -framerate 24 -t 10 -i goldmetal.jpg \
-i 3251.mp3 \
-loop 1 -framerate 24 -t 10 -i cash-register-1885558.jpg \
-loop 1 -framerate 24 -t 10 -i ice-1915849.jpg \
-filter_complex "[0:v][1:a][2:v][3:v]concat=n=4:v=1:a=1[v][a]" -map "[v]" -map "[a]" -c:v libx264 /path/'.$v_title.'.mp4 2>&1');
} ?>
But I'm getting this error --
**Stream specifier ':v' in filtergraph description [0:v][1:a][2:v][3:v]concat=n=4:v=1:a=1[v][a] matches no streams.**
EDIT 3
I almost got it working as needed, using 2 commands, one for the images and fade, the other to combine the audio. The only issue I'm having is changing the time each image shows up. --
echo shell_exec('ffmpeg \
-loop 1 -t 5 -i '.$thepath .'/'.$v_pix1.' \
-loop 1 -t 5 -i ' .$thepath . '/cash-register-1885558.jpg \
-loop 1 -t 5 -i ' .$thepath . '/ice-1915849.jpg \
-loop 1 -t 5 -i '.$thepath .'/'.$v_pix1.' \
-loop 1 -t 5 -i ' .$thepath . '/ice-1915849.jpg \
-filter_complex \
"[0:v]setpts=PTS-STARTPTS,fade=t=out:st=4:d=1,scale=1280:720,setdar=16/9,setsar=sar=300/300[v0]; \
[1:v]setpts=PTS-STARTPTS,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1,scale=1280:720,setdar=16/9,setsar=sar=300/300[v1]; \
[2:v]setpts=PTS-STARTPTS,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1,scale=1280:720,setdar=16/9,setsar=sar=300/300[v2]; \
[3:v]setpts=PTS-STARTPTS,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1,scale=1280:720,setdar=16/9,setsar=sar=300/300[v3]; \
[4:v]setpts=PTS-STARTPTS,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1,scale=1280:720,setdar=16/9,setsar=sar=300/300[v4]; \
[v0][v1][v2][v3][v4]concat=n=5:v=1:a=0,format=yuv420p[v]" -map "[v]" -y '.$thepath .'/fadeout.mp4 2>&1');
echo shell_exec('ffmpeg \
-i '.$thepath .'/fadeout.mp4 \
-i '.$thepath .'/3251.mp3 \
-filter_complex "[0:v:0][1:a:0] concat=n=1:v=1:a=1 [vout] [aout]" -map "[vout]" -map "[aout]" -c:v libx264 -r 1 -y '.$thepath .'/mergetest.mp4 2>&1');

Answer 1

This answer addresses the ffmpeg specific questions in your broad multi-question.
This example will take images of any arbitrary size, fit them into a 1280x720 box, fade between images, and play the audio at the same time. The video will end whenever the images or audio ends: whichever is shortest.
ffmpeg \
-i audio1.mp3 \
-i audio2.wav \
-loop 1 -t 5 -i image1.jpg \
-loop 1 -t 5 -i image2.png \
-loop 1 -t 5 -i image3.jpg \
-filter_complex \
"[2:v]scale=1280:720:force_original_aspect_ratio=decrease,pad=1280:720:(ow-iw)/2:(oh-ih)/2,setsar=1,fade=t=out:st=4:d=1[v1]; \
[3:v]scale=1280:720:force_original_aspect_ratio=decrease,pad=1280:720:(ow-iw)/2:(oh-ih)/2,setsar=1,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1[v2]; \
[4:v]scale=1280:720:force_original_aspect_ratio=decrease,pad=1280:720:(ow-iw)/2:(oh-ih)/2,setsar=1,fade=t=in:st=0:d=1,fade=t=out:st=4:d=1[v3]; \
[0:a][1:a]amerge=inputs=2[a];
[v1][v2][v3]concat=n=3:v=1:a=0,format=yuv420p[v]" \
-map "[v]" -map "[a]" -ac 2 -shortest -movflags +faststart output.mp4