In case anyone is wondering what these filters are for, have a look at the W3C's specification:
http://www.w3.org/TR/PNG-Filters.htmlCes constantes sont définies par cette extension, et ne sont disponibles que si cette extension a été compilée avec PHP, ou bien chargée au moment de l'exécution.
GD_VERSION
    (string)
   GD_MAJOR_VERSION
    (int)
   GD_MINOR_VERSION
    (int)
   GD_RELEASE_VERSION
    (int)
   GD_EXTRA_VERSION
    (string)
   GD_BUNDLED
    (int)
   IMG_AVIF
    (int)
   IMG_BMP
    (int)
   IMG_GIF 
    (int)
   IMG_JPG 
    (int)
   IMG_JPEG 
    (int)
   Note:
Cette constante à la même valeur que
IMG_JPG
IMG_PNG 
    (int)
   IMG_TGA
    (int)
   IMG_WBMP 
    (int)
   IMG_XPM 
    (int)
   IMG_WEBP
    (int)
   IMG_WEBP_LOSSLESS
    (int)
   IMG_COLOR_TILED 
    (int)
   IMG_COLOR_STYLED 
    (int)
   IMG_COLOR_BRUSHED 
    (int)
   IMG_COLOR_STYLEDBRUSHED 
    (int)
   IMG_COLOR_TRANSPARENT 
    (int)
   IMG_AFFINE_TRANSLATE
    (int)
   IMG_AFFINE_SCALE
    (int)
   IMG_AFFINE_ROTATE
    (int)
   IMG_AFFINE_SHEAR_HORIZONTAL
    (int)
   IMG_AFFINE_SHEAR_VERTICAL
    (int)
   IMG_ARC_ROUNDED 
    (int)
   Note:
Cette constante à la même valeur que
IMG_ARC_PIE
IMG_ARC_PIE 
    (int)
   IMG_ARC_CHORD
    (int)
   IMG_ARC_NOFILL 
    (int)
   IMG_ARC_EDGED 
    (int)
   IMG_GD2_RAW 
    (int)
   IMG_GD2_COMPRESSED 
    (int)
   IMG_EFFECT_REPLACE 
    (int)
   IMG_EFFECT_ALPHABLEND 
    (int)
   IMG_EFFECT_NORMAL 
    (int)
   IMG_EFFECT_OVERLAY 
    (int)
   IMG_EFFECT_MULTIPLY 
    (int)
   IMG_FILTER_NEGATE 
    (int)
   IMG_FILTER_GRAYSCALE 
    (int)
   IMG_FILTER_BRIGHTNESS 
    (int)
   IMG_FILTER_CONTRAST 
    (int)
   IMG_FILTER_COLORIZE 
    (int)
   IMG_FILTER_EDGEDETECT 
    (int)
   IMG_FILTER_GAUSSIAN_BLUR 
    (int)
   IMG_FILTER_SELECTIVE_BLUR 
    (int)
   IMG_FILTER_EMBOSS 
    (int)
   IMG_FILTER_MEAN_REMOVAL 
    (int)
   IMG_FILTER_SMOOTH 
    (int)
   IMG_FILTER_PIXELATE
    (int)
   IMG_FILTER_SCATTER 
    (int)
   IMAGETYPE_GIF 
    (int)
   IMAGETYPE_JPEG 
    (int)
   IMAGETYPE_JPEG2000 
    (int)
   IMAGETYPE_PNG 
    (int)
   IMAGETYPE_SWF 
    (int)
   IMAGETYPE_PSD 
    (int)
   IMAGETYPE_BMP 
    (int)
   IMAGETYPE_WBMP
    (int)
   IMAGETYPE_XBM
    (int)
   IMAGETYPE_TIFF_II 
    (int)
   IMAGETYPE_TIFF_MM 
    (int)
   IMAGETYPE_IFF 
    (int)
   IMAGETYPE_JB2 
    (int)
   IMAGETYPE_JPC 
    (int)
   IMAGETYPE_JP2 
    (int)
   IMAGETYPE_JPX 
    (int)
   IMAGETYPE_SWC 
    (int)
   IMAGETYPE_ICO
    (int)
   IMAGETYPE_WEBP
    (int)
   IMAGETYPE_AVIF
    (int)
   IMAGETYPE_UNKNOWN
    (int)
   IMAGETYPE_COUNT
    (int)
   PNG_NO_FILTER 
    (int)
   PNG_FILTER_NONE 
    (int)
   PNG_FILTER_SUB 
    (int)
   PNG_FILTER_UP 
    (int)
   PNG_FILTER_AVG 
    (int)
   PNG_FILTER_PAETH 
    (int)
   PNG_ALL_FILTERS 
    (int)
   IMG_FLIP_VERTICAL
    (int)
   IMG_FLIP_HORIZONTAL
    (int)
   IMG_FLIP_BOTH
    (int)
   IMG_BELL
    (int)
   IMG_BESSEL
    (int)
   IMG_BILINEAR_FIXED
    (int)
   IMG_BICUBIC
    (int)
   IMG_BICUBIC
    (int)
   IMG_BLACKMAN
    (int)
   IMG_BOX
    (int)
   IMG_BSPLINE
    (int)
   IMG_CATMULLROM
    (int)
   IMG_GAUSSIAN
    (int)
   IMG_GENERALIZED_CUBIC
    (int)
   IMG_HERMITE
    (int)
   IMG_HAMMING
    (int)
   IMG_HANNING
    (int)
   IMG_MITCHELL
    (int)
   IMG_POWER
    (int)
   IMG_QUADRATIC
    (int)
   IMG_SINC
    (int)
   IMG_NEAREST_NEIGHBOUR
    (int)
   IMG_WEIGHTED4
    (int)
   IMG_TRIANGLE
    (int)
   IMG_CROP_BLACK
    (int)
   IMG_CROP_DEFAULT
    (int)
   IMG_CROP_TRANSPARENT.
     Avant PHP 7.4.0, la libgd incluse revenait à IMG_CROP_SIDES
     si l'image n'avait pas de couleur transparente.
    
   IMG_CROP_SIDES
    (int)
   IMG_CROP_THRESHOLD
    (int)
   seuil
     et la couleur donnés.
    
   IMG_CROP_TRANSPARENT
    (int)
   IMG_CROP_WHITE
    (int)
   In case anyone is wondering what these filters are for, have a look at the W3C's specification:
http://www.w3.org/TR/PNG-Filters.html